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Pesquisa Veterinária Brasileira

Print version ISSN 0100-736XOn-line version ISSN 1678-5150

Pesq. Vet. Bras. vol.37 no.10 Rio de Janeiro Oct. 2017

http://dx.doi.org/10.1590/s0100-736x2017001000001 

LIVESTOCK DISEASES

Evaluation of the biofilm formation capacity of Pasteurella multocida strains isolated from cases of fowl cholera and swine lungs and its relationship with pathogenicity

Avaliação da capacidade de formação de biofilme por cepas de Pasteurella multocida isoladas de casos de cólera aviária e de pulmões de suínos e sua relação com a patogenicidade

Brunna D. de Emery2  * 

Thales Q. Furian2 

Roberta M. Pilatti2 

Gabriela Z. Chitolina2 

Karen A. Borges2 

Carlos T.P. Salle2 

Hamilton L.S. Moraes2 

2 Centro de Diagnóstico e Pesquisa Aviária (CDPA), Faculdade de Veterinária, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 8824, Porto Alegre, RS 91540-000, Brazil.

ABSTRACT:

Pasteurella multocida is a Gram-negative bacillus that causes economic losses due to the development of respiratory diseases in several animal species. Among the mechanisms of virulence, the formation of biofilms is an important factor for bacterial survival in hostile environments. Studies of biofilm formation by P. multocida are needed because P. multocida is an important pathogen involved in respiratory infections. However, in contrast to other microorganisms, few studies of biofilm formation have examined P. multocida. Studies comparing the pathogenicity of microbial strains as a function of their biofilm production capacity are also rare. Consequently, the aim of this study was to evaluate the biofilm formation capacity of 94 P. multocida strains isolated from cases of fowl cholera and from swine lungs on polystyrene plates. The associations of the biofilm formation capacity with the pathogenicity index (PI) in vivo and with the presence of four genes (screened by PCR) of the tad locus (tadB, tadD, tadE and tadG), described as adhesion markers, were also determined. Strains from both animal origins were able to form biofilms. However, most of the specimens (52.13%) were classified as weak producers, and more than 40% of the strains of P. multocida (40.42%) did not produce biofilms. There was no significant difference (p>0.05) in the degree of biofilm production between the two sources of isolation. Of the analyzed strains, 56.52% contained all four genes (tadB, tadD, tadE and tadG). The PI arithmetic mean of the strains classified as non-biofilm producers was significantly different (p<0.05) from the PI of moderate-producer strains. The PI of specimens classified as weak biofilm producers also differed significantly (p<0.05) from that of the moderate-producer strains. The results indicate that even though the P. multocida strains isolated from cases of fowl cholera and swine lungs formed biofilms on polystyrene surfaces, adhesion was usually weak. The genes tadB, tadD, tadE and tadG were not significantly associated (p>0.05) with the production of biofilms and with the origin of a given strain. Finally, low virulence strains may suggest a higher biofilm formation capacity on polystyrene plates.

INDEX TERMS: Pasteurella multocida; tad locus; biofilm; pathogenicity

RESUMO:

Pasteurella multocida é um bacilo Gram negativo que ocasiona perdas econômicas, geralmente associadas a doenças respiratórias em diversas espécies animais. Entre os mecanismos de virulência existentes, a formação de biofilmes demonstra ser um importante fator para a proteção e para a sobrevivência bacteriana em ambientes hostis. Estudos relacionados à formação de biofilmes por P. multocida são necessários, uma vez que este é um importante patógeno envolvido em infecções respiratórias. Entretanto, ainda são poucos os estudos desenvolvidos nesta área, quando comparados com aqueles envolvendo outros microrganismos. Também são os raros os estudos que comparam a patogenicidade das cepas com a sua capacidade de produção de biofilme. Neste contexto, o objetivo deste estudo foi avaliar a capacidade de formação de biofilme em placas de poliestireno de 94 cepas de P. multocida isoladas de casos de cólera aviária e de pulmões de suínos, associando-se com o índice de patogenicidade (IP) in vivo e com a presença de quatro genes do locus tad (tadB, tadD, tadE e tadG), descritos como marcadores de adesão e pesquisados através de PCR. As cepas de ambas as origens foram capazes de formar biofilme. Contudo, a maioria dos exemplares (52,12%) foi classificada como fracamente produtora e mais de 40% das cepas de P. multocida (40,42%) não produziram biofilme. Não foi observada diferença estatística (p>0,05) quanto ao grau de produção de biofilme entre as duas origens de isolamento. 56,52% das cepas analisadas apresentaram os quatro genes (tadB, tadD, tadE e tadG) concomitantemente. O IP médio das cepas classificadas como não produtoras de biofilme apresentou diferença estatística (p˂0,05) em relação ao IP das cepas moderadamente produtoras. Os exemplares classificados como fracamente produtores de biofilme diferiram significativamente (p˂0,05) do grupo de cepas moderadamente produtoras. Os resultados obtidos indicaram que, apesar de as cepas de P. multocida isoladas de casos de cólera aviária e do pulmão de suínos apresentarem capacidade de formar biofilme em superfícies de poliestireno, a adesão ocorreu geralmente de forma fraca. Os genes tadB, tadD, tadE e tadG, pertencentes ao locus tad, não apresentaram associação significativa com a produção de biofilme e nem com a origem de isolamento da cepa. Por fim, observou-se que as cepas de menor patogenicidade apresentaram uma maior capacidade de formação de biofilme em placas de poliestireno.

TERMOS DE INDEXAÇÃO: Pasteurella multocida; locus tad; biofilme; patogenicidade

Introduction

Pasteurella multocida is a Gram-negative bacillus that causes economic losses associated with numerous respiratory diseases in poultry farms (Moraes et al. 2014) and, in some cases, the processing of animal products (Ribeiro et al. 2012). P. multocida causes fowl cholera (FC) as well as progressive atrophic rhinitis, pneumonia and serositis in swine, hemorrhagic septicemia in cattle and pasteurellosis in rabbits (Boyce et al. 2010, Ferreira et al. 2012, Wilkie et al. 2012). Sporadic cases of infections in humans are usually associated with scratches and bites caused by dogs and cats (Hunt Gerardo et al. 2001, Kawashima et al. 2010). These infections can range from chronic to fatal sepsis, as is generally observed in the cases of FC and hemorrhagic septicemia (Wilkie et al. 2012).

Among various virulence mechanisms, the formation of biofilms is an important factor for bacterial survival and protection in hostile environments, such as in tissue or on an inert surface, with exposure to light, desiccation, osmotic pressure and to pH and temperature variations (Costerton et al. 1995, 1999, Sutherland 1997). A biofilm is a structured community of bacterial cells enclosed in a self-produced polymeric extracellular matrix that is attached to a biotic or abiotic surface (Costerton et al. 1995, Donlan & Costerton 2002, Hall-Stoodley & Stoodley 2009). Infections involving sessile microorganisms are often chronic and difficult to treat (Costerton et al. 1999). The microbe’s ability to escape the host immune system under these conditions and to resist high antimicrobial concentrations facilitates the persistence of the microorganism (Lemon et al. 2008, Silva et al. 2014).

The National Institutes of Health (NIH) estimates that 75% of infections in humans result from the formation and persistence of biofilms (Richards & Melander 2009). Considering the extensive role of biofilms in human infection, it is probable that biofilms are also related to a variety of infections in animals (Clutterbuck et al. 2007, Freeman et al. 2009). The major bacteria with known potential for biofilm formation include representatives of the genera Pseudomonas, Vibrio, Salmonella and E. coli (Freeman et al. 2009, Rajagopal et al. 2013). Recent studies suggest that P. multocida can also form biofilms, including in swine lungs (Ross 2006, Rajagopal et al. 2013, Moraes et al. 2014). However, such studies are rare compared with those involving other organisms. In addition, biofilm formation is usually not compared to the pathogenicity of a given isolate.

The main structures associated with virulence that have been identified in P. multocida strains are the capsule and lipopolysaccharide (LPS). However, other factors are known to be important virulence markers in P. multocida (Wilkie et al. 2012). These factors include genes encoding outer membrane proteins (ompH, oma87), genes related to iron metabolism (hgbA, hgbB, exBD-tonB) and those encoding fimbriae and adhesins (ptfA, pfhA, tad) (Ewers et al. 2006, Corney et al. 2007, Hatfaludi et al. 2010). The genes of the tad locus have been described as bacterial adhesion markers. Homologous regions of the biosynthesis locus play an important role in biofilm formation, colonization and pathogenesis of other Pasteurellaceae members and some Gram-negative bacteria (Tomich et al. 2007, Moraes et al. 2014).

Because P. multocida is an important pathogen involved in respiratory infections, studies of biofilm formation are necessary. Biofilms can be an important virulence factor for bacterial survival in the host (Rajagopal et al. 2013). Consequently, the aim of this study was to evaluate the biofilm formation capacity of P. multocida isolates in vitro and the relationship of biofilm formation capacity with pathogenicity, as well to detect genes of the tad locus in these strains.

Materials and Methods

Pasteurella multocida samples. For this study, we selected 94 strains isolated from clinical cases of FC (n=55) and from swine lungs of healthy animals (n=39) at slaughter. All strains were stored in total sheep blood at a temperature of -80°C. Reactivation and preliminary tests for the confirmation of the purity of Pasteurella multocida samples were performed as described by Glisson et al. (2008) and Furian et al. (2013).

The strains were reactivated in brain heart infusion (Brain Heart Infusion - BHI - Oxoid; Basingstoke, Hampshire, UK) broth and incubated at 37°C for 24 hours. After this period, the strains were plated on blood agar (Oxoid) supplemented with 5% defibrinated sheep’s blood and on MacConkey agar (Oxoid), a culture medium in which P. multocida growth does not occur. The strains were then incubated again at 37°C for 24 hours. Subsequently, the morphology of the colonies present on blood agar was evaluated. Giemsa staining was used to observe the bipolar characteristics of the bacterial cells. Finally, evaluation of catalase and oxidase, in addition to biochemical tests using the Bactray III system (Laborclin; Pinhais, Paraná, Brazil), were performed. One colony was selected for DNA extraction using the NucleoSpin Tissue kit (Macherey-Nagel, Düren, Germany). A PCR protocol for species-specific amplification of a 460-bp fragment of the kmt gene was performed, as described by Townsend et al (1998). Reference strains of P. multocida (ATCC 15742, ATCC 12945) were selected as positive controls.

Evaluation of biofilm formation capacity at 37°C. The methodology used was adapted from the studies of Stepanovic et al. (2000), Silva et al. (2014) and Borges (2015).

Initially, the strains were plated on TSA agar without glucose (Trypticase Soy Agar, Merck, Darmstadt, Germany) and incubated at 37°C for 24 hours. Subsequently, a colony of each sample was inoculated in TSB broth without glucose (Trypticase Soy Broth - TSB - Difco; Detroit, USA) and incubated at 37°C for 24 hours. After this period, the culture was diluted in TSB broth to achieve a concentration corresponding to 1 McFarland scale unit (Probac; São Paulo, Brazil), which indicates a concentration of approximately 3x108 CFU/mL. Then, 200μL of the bacterial suspension from each sample was inoculated in triplicate in polystyrene 96-well flat-bottomed plates (Kasvi, Curitiba, PR, Brazil), including the strains selected as positive controls, and the plates were incubated at 37°C for 24 hours. One strain of Salmonella enteritidis and one strain of Escherichia coli in our lab previously classified as strong biofilm producers were selected as positive controls. The negative control was TSB broth without glucose, which was added to three wells of each plate.

After incubation, the bacterial suspensions were removed, and the plates were washed three times with 250 μL of sterile 0.9% sodium chloride solution to remove planktonic cells. Thereafter, 200μL of methanol (Nuclear; Diadema, SP, Brazil) was added to fix the adherent bacterial cells. The contents of the wells were removed after 15 minutes, and the plates were dried at room temperature. A 200-μL aliquot of 2% Hucker crystal violet solution was added to stain the fixed cells. After 5 minutes, the plates were washed in water and dried again at room temperature. To perform the reading, 200μL of 33% glacial acetic acid (Nuclear) was added, and the absorbance was measured 1 hour later by an ELISA reader at 550nm (Biotek; Winooski, USA). The absorbance value of each strain (DOa) was obtained by calculating the arithmetic mean of the triplicate wells. The cutoff point was defined as three deviations above the absorbance mean of the negative control (DOc). To determine biofilm formation capacity, the following classification was used: non-adherent (DOa≤DOc), weakly adherent (DOc<DOa≤2xDOc), moderately adherent (2xDOc<DOa≤4xDOc) and strongly adherent (4xDOc<Doa).

Strain pathogenicity index (PI). The pathogenicity indices (PIs) of the 94 strains were associated with biofilm formation capacity at 37°C. The PIs were previously established by Pilatti et al. (2016). In this preliminary study, the strains were classified into three groups of pathogenicity (high, intermediate, low) on a scale from 0 to 10, calculated from the experimental inoculation of one-day-old chicks. The high-pathogenicity group comprised strains with PIs ranging from 8 to 10, the intermediate-pathogenicity group had PIs between 4 and 7, and the low-pathogenicity group included strains with indices from 0 to 3 (Pilatti et al. 2016).

DNA extraction and detection of tad locus genes. An aliquot of 1 mL of BHI broth inoculated overnight with each strain was used for DNA extraction using the commercial extraction kit NucleoSpin (Macherey Nagel; Düren, Germany). The extracted DNA was stored at -2 °C until analysis by PCR to detect the four genes of the tad locus (tadB, tadD, tadE e tadG). The oligonucleotides used were obtained from previous studies (Table 1)”

Table 1. Sequences of tad gene oligonucleotides and the respective amplicon sizes 

Gene Primers sequence (5’- 3’) Amplicon Reference
tadB TTCGCCTAATTGTCCCGTTA 150 bp Moraes et al. (2014)
TGGAAGTTAGGGCAATACCG
tadD TCTACCCATTCTCAGCAAGGC 416 bp Tang et al. (2009)
ATCATTTCGGGCATTCACC
tadE TGGATTCGTCCCAAGAGAAC 195 bp Moraes et al. (2014)
ATCTCTCCTACGGGGAGTCG
tadG AACTTGCCCAATTGTTCTCG 224 bp Moraes et al. (2014)
CCTTCTGGTTGGACTTCTGC

To amplify the tadD gene, we used the PCR protocol described by Furian et al. (2016). The amplification of the tadB, tadE and tadG genes was adapted from the protocol described by Moraes et al. (2014). The reaction mix was composed of 2. μL of 10X buffer, 0. μL of dNTPs (1 mM - Ludwig Biotec; Alvorada, Rio Grande do Sul, Brazil), 0. μL of primers (2 pmol - Invitrogen; Carlsbad, CA, USs), 1.5 U of GoTaq® Hot Start Polymerase (Promega; Madison, Wisconsin, USs), 1.2 μL of MgCl2 (2 mM - Promega) and 17.5 μL of ultra-pure water.

The amplification reactions were performed in a Swift MaxPro thermocycler (Esco Technologies; Singapore) under the following conditions: initial denaturation at 9 °C for 5 minutes followed by 30 cycles of denaturation at 9 °C (30 seconds), annealing at 5 °C (30 seconds) and extension at 7 °C (60 seconds), and a final extension step at 7 °C (10 minutes). Electrophoresis of the amplified products was performed in agarose gels stained with 1.5% ethidium bromide. The gels were photo-documented (Alpha Innotech; San Leandro, California, USs) and then interpreted. The standard strains P. multocida ATCC 12945 and Avibacterium gallinarum ATCC 13360 were selected as positive and negative controls, respectively.

Statistical analysis. Descriptive statistical analysis was used to determine the absolute and relative frequency of virulence genes as well as the grouping of the samples according their biofilm formation capacity. The non-parametric chi-square (χ2) and Fisher’s exact tests were used to analyze the distribution of the strains classified in each biofilm group. The nonparametric Kruskal-Wallis and Mann-Whitney tests were used to compare the means of the pathogenicity indices of strains with their biofilm formation capacity. The Statistical Package for Social Sciences (SPSS) was used for statistical analysis, adopting as a reference a significance level of 5% and a confidence level of 95%.

Results

The Pasteurell. multocida strains isolated from both sources formed biofilms on polystyrene plates, as shown in Table 2. However, most of the specimens (52.13%) were classified as weakly adherent, and more than 40% of the strains (40.42%) did not produce biofilms. Although only swine strains were classified as moderate producers (Table 2), there was no significant difference (p>0.05) in the degree of biofilm production between the two isolate sources”.

Table 2. Classification of the 94 strains of Pasteurella multocida isolated from poultry and swine according to the degree of biofilm production at 37°C 

Biofilm production classification Avian (n=55) Swine (n=39)
non-biofilm producers 25 (45.45%) 13 (33.33%)
weak biofilm producers 29 (52.73%) 20 (51.28%)
moderate biofilm producers 0 (0.00%) 6 (15.39%)
strong biofilm producers 1 (1.82%) 0 (0.00%)
all biofilm producers 30 (54.54%) 26 (66.67%)

All PCR protocols were specific for the detection of tad locus genes. Of the 94 strains of P. multocida, 56.52% possessed all four genes analyzed (tadB, tadD, tadE and tadG).

Analysis of the frequencies of the genes according to the origin of isolation (Table 3) revealed that more than 80% of the avian and swine strains were positive for tadB and 100% were positive for tadE and tadG, regardless of source. In contrast, the tadD gene was detected in 50.91% of avian strains and 82.05% of the strains isolated from swine”.

Table 3. Absolute and relative frequency (%) of the four virulence genes (tadB, tadD, tadE and tadG) according to the origin of the Pasteurella multocida strain 

Origin Virulence genes
tadB tadD tadE tadG
Avian (n=55) 46 (83.64) 28 (50.91) 55 (100%) 55 (100%)
Swine (n=39) 34 (87,18) 32 (82.05) 39 (100%) 39 (100%)

Based on the relative frequencies of the four genes studied, the relationship between the presence of these genes and bacterial adhesion was established. Of the avian strains positive for the four genes screened, 61.53% were classified as biofilm producers, including a strong producer strain. Among the strains isolated from swine, 57.68% of the strains that were positive for the four genes were classified as biofilm producers, including three classified as moderate producers. However, the absence of individual tadB and tadD genes was not significantly associated (p>0.05) with the variation in the biofilm formation capacity of the P. multocida strains.

Following the evaluation of biofilm formation and grouping of the samples, the analyzed strains were compared to their respective PIs (Table 4), which were previously obtained by an in vivo study (Pilatti et al. 2016). The mean PI of the P. multocida strains classified as non-biofilm producers differed significantly (p˂0.05) from the mean PI of moderate biofilm producer strains (Fig 1). Similarly, the PIs of specimens classified as weak biofilm producers differed significantly from PIs of the moderate producer strains. However, there was no significant difference (p>0.05) between the PIs of the strains that did not produce biofilms and the PIs of weak biofilm producers (Fig 1).

Table 4. Classification of the samples according to their biofilm formation, pathogenicity index and pathogenicity group 

Sample Host Biofilm production classification Pathogenicity index (PI)* Groups of Pathogenicity*
1 Chicken Non-biofilm producer 3.27 Low
2 Chicken Non-biofilm producer 6.93 Intermediate
3 Chicken Non-biofilm producer 5.07 Intermediate
4 Chicken Non-biofilm producer 4.8 Intermediate
5 Chicken Non-biofilm producer 6.43 Intermediate
6 Chicken Non-biofilm producer 5.46 Intermediate
7 Chicken Non-biofilm producer 4.88 Intermediate
8 Chicken Non-biofilm producer 6.56 Intermediate
9 Chicken Non-biofilm producer 10 High
10 Chicken Non-biofilm producer 10 High
11 Chicken Non-biofilm producer 10 High
12 Chicken Non-biofilm producer 10 High
13 Chicken Non-biofilm producer 10 High
14 Chicken Non-biofilm producer 8.51 High
15 Chicken Non-biofilm producer 10 High
16 Chicken Non-biofilm producer 10 High
17 Chicken Non-biofilm producer 8.69 High
18 Chicken Non-biofilm producer 10 High
19 Chicken Non-biofilm producer 10 High
20 Chicken Non-biofilm producer 10 High
21 Chicken Non-biofilm producer 8.6 High
22 Chicken Non-biofilm producer 9.18 High
23 Chicken Non-biofilm producer 8.93 High
24 Chicken Non-biofilm producer 10 High
25 Chicken Non-biofilm producer 7.75 High
26 Chicken Weak biofilm producer 2.29 Low
27 Chicken Weak biofilm producer 3.31 Low
28 Chicken Weak biofilm producer 2.17 Low
29 Chicken Weak biofilm producer 1.33 Low
30 Chicken Weak biofilm producer 4.43 Intermediate
31 Chicken Weak biofilm producer 7.44 Intermediate
32 Chicken Weak biofilm producer 3.95 Intermediate
33 Chicken Weak biofilm producer 5.63 Intermediate
34 Chicken Weak biofilm producer 4.69 Intermediate
35 Chicken Weak biofilm producer 5.17 Intermediate
36 Chicken Weak biofilm producer 4.39 Intermediate
37 Chicken Weak biofilm producer 4.56 Intermediate
38 Chicken Weak biofilm producer 7.36 Intermediate
39 Chicken Weak biofilm producer 7.9 High
40 Chicken Weak biofilm producer 7.8 High
41 Chicken Weak biofilm producer 7.57 High
42 Chicken Weak biofilm producer 8.26 High
43 Chicken Weak biofilm producer 9.13 High
44 Chicken Weak biofilm producer 10 High
45 Chicken Weak biofilm producer 7.82 High
46 Chicken Weak biofilm producer 10 High
47 Chicken Weak biofilm producer 7.93 High
48 Chicken Weak biofilm producer 10 High
49 Chicken Weak biofilm producer 8.06 High
50 Chicken Weak biofilm producer 8.87 High
51 Chicken Weak biofilm producer 8.44 High
52 Chicken Weak biofilm producer 7.69 High
53 Chicken Weak biofilm producer 8.13 High
54 Chicken Weak biofilm producer 10 High
55 Chicken Strong biofilm producer 2.99 Low
56 Swine Non-biofilm producer 0.33 Low
57 Swine Non-biofilm producer 2.42 Low
58 Swine Non-biofilm producer 2.25 Low
59 Swine Non-biofilm producer 5.81 Intermediate
60 Swine Non-biofilm producer 4.93 Intermediate
61 Swine Non-biofilm producer 5.44 Intermediate
62 Swine Non-biofilm producer 5.24 Intermediate
63 Swine Non-biofilm producer 3.79 Intermediate
64 Swine Non-biofilm producer 4.73 Intermediate
65 Swine Non-biofilm producer 3.5 Intermediate
66 Swine Non-biofilm producer 3.98 Intermediate
67 Swine Non-biofilm producer 3.6 Intermediate
68 Swine Non-biofilm producer 6.11 Intermediate
69 Swine Weak biofilm producer 3.12 Low
70 Swine Weak biofilm producer 2.1 Low
71 Swine Weak biofilm producer 2 Low
72 Swine Weak biofilm producer 1.17 Low
73 Swine Weak biofilm producer 1.75 Low
74 Swine Weak biofilm producer 2.6 Low
75 Swine Weak biofilm producer 1.33 Low
76 Swine Weak biofilm producer 1.33 Low
77 Swine Weak biofilm producer 1.67 Low
78 Swine Weak biofilm producer 6.77 Intermediate
79 Swine Weak biofilm producer 4,89 Intermediate
80 Swine Weak biofilm producer 7,5 Intermediate
81 Swine Weak biofilm producer 6,31 Intermediate
82 Swine Weak biofilm producer 6,65 Intermediate
83 Swine Weak biofilm producer 6,42 Intermediate
84 Swine Weak biofilm producer 7,2 Intermediate
85 Swine Weak biofilm producer 7,25 Intermediate
86 Swine Weak biofilm producer 10 High
87 Swine Weak biofilm producer 10 High
88 Swine Weak biofilm producer 8,27 High
89 Swine Moderate biofilm producer 1,25 Low
90 Swine Moderate biofilm producer 1,33 Low
91 Swine Moderate biofilm producer 1,42 Low
92 Swine Moderate biofilm producer 2,5 Low
93 Swine Moderate biofilm producer 1,58 Low
94 Swine Moderate biofilm producer 5,26 Intermediate

* Pillatti et al. (2016)

Fig.1 Distribution of Pasteurella multocida strains of avian and swine origin according to their biofilm formation and to their pathogenicity index (PI) calculated from the experimental inoculation of chicks. 

Discussion

Several pathogenic bacteria present in animals produce biofilms (Costerton et al. 1999). However, studies of the production of these structures by Pasteurell multocida strains are rare (Olson et al. 2002, Romanò et al. 2013). Biofilm-forming bacteria adhere to practically all surfaces in vitro under suitable conditions. Among these conditions, the restriction of nutrients, such as TSB broth without glucose (used in the present study), is an important condition for the formation of biofilms (Christensen et al. 1982, Dewanti & Wong 1995). Furthermore, Olson et al. (2002) determined that TSB broth supplementation with 2% of fetal bovine serum and incubation in 10% CO2 are necessary conditions for P. multocida biofilm formation. Such conditions should be evaluated in future studies for comparison with the results obtained here.

Several substrates, both hydrophobic and hydrophilic, are used to stimulate the fixation of bacteria in vitro (Rajagopal et al. 2013). The polystyrene selected in this study is an inert material often used to assess biofilm formation capacity in bacterial species (Silva et al. 2014, Borges 2015, Narayanan et al. 2016, Salimena et al. 2016). A recent study by Rajagopal et al. (2013) also reported the adhering ability of P. multocida to an inert surface formed by bentonite clay.

The potential of P. multocida strains to form biofilms was observed in this study by growing the organism under nutrient restriction and on polystyrene plates. Regardless of origin, the strains analyzed exhibited low levels of adhesion without strong biofilm production, as described for other bacteria, such as Salmonella spp. (Marquezini 2015) and Pseudomonas aeruginosa (Hassett et al. 2009). However, P. multocida did exhibit the ability to form biofilms. Thus, research on the adhesion capacity of microorganisms is essential because P. multocida is an important respiratory pathogen and the production of these structures could contribute to the increased virulence of the organism (Rajagopal et al. 2013).

It is also important to note that there are different ways to evaluate the production of biofilms, including the congo red agar method, bioluminescent assay, air-liquid interface, microscopy examination, among others (Freeman et al. 1989, Donlan et al. 2001, Hassan et al. 2011, Cabarkapa et al. 2015, Karched et al. 2015). A clear picture of attachment cannot be obtained without considering the effects of the substratum, conditioning films forming on the substratum, hydrodynamics of the aqueous medium, characteristics of the medium and various properties of the cell surface (Donlan 2002). Thus, the use of different quantitative or qualitative assessment methods may present different results, especially with P. multocida, and should be compared in the future.

The degree of biofilm formation was compared with the PI obtained for the same strain, which can be considered a measure of the in vivo pathogenicity of the isolate (Pilatti et al. 2016). Pathogenicity is related to the characteristics of a microorganism that are involved in the capacity to cause infection, whereas virulence determines the level of disease caused by a specific pathogenic microorganism (Kubatzky 2012). However, the low biofilm formation capacity of the pathogenic P. multocida strains examined in this study suggests that virulence factors are predominant in the process of infection. For example, the main P. multocida surface components, lipopolysaccharide and capsule, play important roles in the resistance to phagocytosis and the inhibition of complement and the activity of antimicrobial peptides (Wilkie et al. 2012). In addition, other virulence factors are involved in the survival of the microorganism in the infected host, and pathogenic bacteria generally have multiple adhesins to distinct cell types (Harper et al. 2006, Kline et al. 2009, Wilson & Ho 2013).

Biofilm formation may also function as a survival mechanism under adverse conditions in less virulent strains or with P. multocida involved in secondary infections. For example, P. multocida can colonize swine lungs in the form of biofilms, causing lesions that are generally difficult to treat because of the blockade of antimicrobial action and the monocyte phagocytic system (Donlan & Costerton 2002, Moraes et al. 2014). Despite these results, the correlation of biofilm formation capacity with the pathogenicity of a given strain should be further evaluated in future studies with a larger number of strains classified as strong biofilm producers. This relationship was not possible to evaluate statistically in this study due to the presence of only a single strong biofilm producing strain.

In addition, the lesions evaluated through the experimental inoculation (Pilatti et al. 2016) involve mechanisms and virulence factors previously surveyed (Furian et al. 2016) that are not directly related to biofilm formation. For example, the widespread petechiae on serosal and epicardial surfaces of the inoculated animals were an indication of the consumptive coagulopathy common to endotoxemia (Boyce et al. 2010). Likewise, death after inoculation is probably associated with a massive bacteremia and endotoxic shock with the lysis and subsequent release of endotoxins that damage the host tissues (Harper et al. 2006). On the other hand, bacteria in biofilms can be viewed as biogenic habitat formers at a microscale (Flemming et al. 2016) and they are not directly related to these lesions. By generating a matrix, bacteria in biofilms create a physically distinct habitat that provides shelter, promotes the accumulation of nutrients and fundamentally alters both the physicochemical environment and interactions among the organisms therein (Flemming et al. 2016).

The genes of the tad locus belong to a group of putative adhesins (Hatfaludi et al. 2010). The Tad system, which is present in many bacterial genera, is the main subtype of the type III secretion system, and related genes encode proteins necessary for the formation of Flp fimbriae, which can be essential for biofilm formation (Tomich et al. 2007, Hatfaludi et al. 2010). Among the four genes selected for analysis, tadE and tadG were detected in 100% of FC and swine lung isolates. In contrast, the frequency of the tadB and tadD genes was variable depending on the source. Similarly, Moraes et al. (2014) detected the tadA, tadB, tadC, tadE, tadF and tadG genes in 100% of isolates from swine lungs, except for tadD, as reported in the current study.

However, the presence or the absence of the gene establishes a linear relationship among samples, which may not be true, since the expression and the interaction are not considered. Furthermore, several genetic factors participate in biofilm formation (Beloin & Ghigo 2005), which is influenced by environmental factors, such as pH, temperature and concentration of nutrients in the medium (Steenackers et al. 2012, Flemming et al 2016). Thus, these genetic factors, especially when they are involved in the early stages of biofilm formation, can be functionally replaced or overridden by others, depending on the media and growth conditions. Similarly, finding common bacterial biofilm gene-expression patterns through global expression analysis remains difficult (Beloin & Ghigo 2005).

The tadD gene encodes an adhesin with a non-specific adhesion pattern (May et al. 2001) and was previously investigated in genetic profile analysis studies of P. multocida strains isolated from swine, cattle and poultry (Tang et al. 2009, Katsuda et al. 2013, Moraes et al. 2014, Furian et al. 2016). These studies also highlight the presence of variations in the gene and its positive association with P. multocida serotype A (Tang et al. 2009) and negative association with serogroup D, as previously observed in a study by our group (Furian et al. 2016).

Although tadD is in a locus associated with the formation of biofilms, including the presence of biofilms in lung lesions in swine and cattle (Khamesipour et al. 2014, Moraes et al. 2014), the presence of tadD in individual strains in the current study was not significantly associated with biofilm formation capacity. This finding is probably explained by the presence of other genes of the same locus related to the formation of biofilms and mutations in the tadD gene that interfere with the adhesion potential of P. multocida (Fuller et al. 2000).

Conclusions

The evaluation of the biofilm formation capacity of Pasteurella multocida strains isolated from cases of FC and swine lung after slaughter demonstrated that the adhesion of these strains was usually weak, despite their ability to form biofilms on polystyrene surfaces.

The tadB, tadD, tadE and tadG genes of the tad locus were simultaneously detected in most of the analyzed strains, and a significant association was not observed (p>0.05) between the presence of these genes and the origin of the strain.

Similarly, there was no association (p>0.05) between the presence of the tadB or tadD genes and the biofilm formation capacity of the strains. Finally, low virulence strains may suggest a higher capacity for biofilm formation on polystyrene plates.

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Received: July 28, 2016; Accepted: December 23, 2016

*Corresponding author: thales.furian@ufrgs.br

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