Frequency and antimicrobial susceptibility of Shigella species isolated in Children Medical Center Hospital, Tehran, Iran, 2001-2006

ORIGINAL ARTICLE

Frequency and antimicrobial susceptibility of Shigella species isolated in Children Medical Center Hospital, Tehran, Iran, 2001-2006

Babak Pourakbari^I,II; Setareh Mamishi, MD^II; Negar Mashoori^III; Nastaran Mahboobi^III; Mohammad H Ashtiani^IV; Shahla Afsharpaiman^V; Masomeh Abedini^VI

^IPediatrics Infectious Diseases Research Center, Tehran University of Medical Sciences, Tehran, Iran

^IIDepartment of Infectious Disease, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran

^IIISchool of Medicine, Tehran University of Medical Sciences, Tehran, Iran

^IVDepartment of Pathology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran

^VDepartment of Pediatrics, School of Medicine, Sanandaj University of Medical Sciences, Tehran, Iran.

^VIDepartment of Pediatrics, School of Medicine, Baghiatallah University of Medical Sciences, Tehran, Iran

^{Correspondence}

ABSTRACT

Appropriate antimicrobial treatment of shigellosis depends on identifying its changing resistance pattern over time. We evaluated 15,255 stool culture submitted from July 2001 to June 2006 to the Laboratory of Children Medical Center Hospital. Specimen culture, bacterial identification, and disk diffusion susceptibility testing were performed according to National Committee for Clinical Laboratory Standards guidelines. From 15,255 stool samples, 682 (4.5%) were positive for Shigella species. The most common species of Shigella were S. flexneri (48%) and S. sonnei (45%); other results were S. dysenteriae (5%) and S. boydii (2%). The rate of Sensitivity to ceftriaxone (95%), ceftizoxime (94%), and nalidixic acid (84%) were among our isolates. Resistance to co-trimoxazole and ampicillin was 87% and 86%, respectively. S. flexneri was more multiresistant than other species (47.9%). Our isolates are overall most sensitive to ceftriaxone, ceftazidime, and nalidixic acid (> 84%). They were most resistant to co-trimoxazole and ampicillin (> 86%). Because resistance varies according to specific location, continuous local monitoring of resistance patterns is necessary for the appropriate selection of empirical antimicrobial therapy.

Keywords: Shigella, antimicrobial susceptibility, dysentery.

INTRODUCTIO N

Dysentery caused by Shigella species is called shigellosis, which is one of the most common causes of dysentery in children. It is responsible for increased cases of morbidity and mortality in developing countries. Global studies suggest there are 164.7 million episodes of shigellosis per year, of which 163.2 million were in devel oping countries, and 1.5 million in developed countries. These episodes result in 1.1 million deaths, of which two-thirds occur in children under 5 years of age.¹

The epidemiology and antibiotic susceptibility of Shigella species changes over time. Treatment of shigellosis by appropriate antimicrobial agents has proven to be effective in shortening the duration of fever, diarrhea and toxemia, and apparently in reducing the risk of lethal complications as well. Also, the excretion of pathogen in stool is shortened significantly, reducing the spread of infection.2 Appropriate antibiotic treatment of shigellosis depends on identifying resistance patterns. So, updated knowledge of Shigella susceptibility is necessary for appropriate empirical antibiotic treatment. In this study, we assessed the frequency of Shigella species and their antimicrobial resistance at Children's Medical Center (CMC) Hospital between July 2001 and June 2006. The aims of this study were to analyze antimicrobial resistance of Shigella isolates to suggest timely recommendations for empirical antibiotic therapy.

MATERIAL AND METHODS

Source of specimens

We evaluated 15,255 stool culture submitted from July 2001 to June 2006 to the Laboratory of CMC Hospital. Only one Shigella isolate per patient per diarrheal episode was included in the analysis. All the included patients lived in Tehran at the time of study and they were not of pathogen in stool is shortened significantly, hospitalized or received any antibiotic during -the month before presentation with shigellosis. In addition to being a referral tertiary care cen tre, CMC Hospital is one of the educational hospitals of Tehran University of Medical Sciences.

Microbial examination

Stool specimens were cultured on Salmonella-Shigella (SS) Agar and Eosin methylene blue (EMB) agar and incubated at 37º C for 1 to 5 days. Biochemical tests were done by standard methods on grown bacteria to identify Shigella species, which grouped serologically by slide agglutination with specific antisera. Antibiotic susceptibility to ceftazidime, tobramycin, ceftizoxime, ceftriaxone, chloramphenicol, kanamycin, nalidixic acid, cephalothin, co-trimoxazole, ampicillin, gentamicin and amikacin was determined in all specimens by disc diffusion method according to the National Committee for Clinical Laboratory Standards (NCCLS).³ To verify that susceptibility test results were accurate, we used E. coli ATCC 25922 as control strain according NCCLS guideline.³ The antimicrobial agents selected for analysis were those commonly included in the treatment of shigellosis in Iran. Results were interpreted according to NCCLS guidelines as either sensitive, intermediate, or resistant.⁴ In our study, we considered both intermediate and resistant as resistant. Isolates were considered multiresistant if they were resistant to ampicillin and co-trimoxazole.⁵ Antimicrobial susceptibility results were rounded down if < 0.5 and were presented as whole numbers if > 0.5.

Statistical analysis

The significance of differences in proportions of antimicrobial resistance of Shigella species was determined by the chi-square test or the Fisher Exact test (when the expected value in > 20% of the cells was < 5). P-value < 0.05 was considered statistically significant. Statistical calculations were performed with SPSS statistical software (version 13.0; SPSS Inc., Chicago, IL, USA).

RESULTS

From 15,255 stool samples, 682 (4.5%) were positive for Shigella species. The species of 397 out of 682 Shigella isolates were determined. The most common species of Shigella was S. flexneri (48%) and S. sonnei (45%), and other results were S. dysenteriae (5%) and S. boydii (2%) (Table 1). Shigella isolates were totally most sensitive to ceftriaxone (95.4%), ceftizoxime (94%), and nalidixic acid (83.5%); and most resistant to co-trimoxazole (87%) and ampicillin (86%). All results are shown in Table 2. S. flexneri was most sensitive to Ceftriaxone (94%), Ceftizoxime (90%), and Nalidixic acid (89%); and most resistant to Ampicillin (96%) and Co-trimoxazole (90%). Comparing the results of susceptibility by year over the study period, it was found increase of rates of resistance to cefazolin, cefixime, tobramycin, amikacin in S. flexneri isolates (p-value < 0.05). S. sonnei was most sensitive to ceftizoxime (98%), ceftriaxone (97%), and chlaromphenicol (93%); and most resistant to co-trimoxazole (92%) and ampicillin (73%). These isolates comparing the results of susceptibility by year showed increase of rates of resistance to kanamycin, cefalotin, ampicillin, gentamycin, amikacin, ceftriaxone, ceftazidime, tobramycin (p-value < 0.05). S. dysenteriae was completely sensitive to ceftriaxone, ceftizoxime, cephalotine, co-trimoxazole (100%), and it was most resistant to kanamycin (100%), amikacin (63%), and nalidixic acid (63%). S. boydii was totally sensitive to gentamicin, amikacin, ceftriaxone, ceftizoxime, ceftazidime, and tobramycin (100%); and most resistant to co-trimoxazole (75%), cephalotine (75%), ampicillin (63%), and kanamycin (60%) (Table 3). In S. dysenteriae and S. boydii isolates were not seen any valuable significance in rate of resistance when comparing the results by the years. Among 682 isolates, 500 isolates were multiresistant (74%). S. flexneri was more multiresistant than other species (Table 4).

DISCUSSION

From the epidemiologic perspective, Shigella is a pathogen that persists as a major public health problem in developing countries, causing treatment center visits, hospitalizations, and deaths, although it also remains an intermittent cause of morbidity and mortality in high risk groups in industrialized countries.

S. sonnei is the predominant Shigella species isolated in developed countries,2,6,⁷ whereas in developing countries and low socio-economic conditions, S. flexneri predominates.^1,2,6,8-1ºIn our study, S. flexneri (48%) was the most common, followed by S. sonnei (45%). Other species were rare (S. dysenteriae and S. boydii 5% and 2%, respectively).

Our isolates are overall most sensitive to ceftriaxone, ceftazidime and nalidixic acid (> 84%). They were most resistant to co-trimoxazole and ampicillin (> 86%) which was compatible with other studies.1,2,⁵

Antibiotic susceptibility results are to some extent different among different species. S. flexneri, the most common species, was most sensitive to ceftriaxone, ceftizoxime, and nalidixic acid; and S. sonnei was most sensitive to ceftizoxime, ceftriaxone, and chloramphenicol. S. flexneri was more sensitive to nalidixic acid than S. sonnei (p-value < 0.05) and S. sonnei was more sensitive to cephalosporines (ceftizoxime, ceftriaxone, ceftazidime, cephalotine) than S. flexneri (p-value < 0.05). It shows that for an empiric therapy, when the suspected species is S. flexneri, nalidixic acid will be more effective and its resistance trend over the study period had not valuable significance, while third cephalosporines will be more effective than nalidixic acid in treatment of shigellosis due to S. sonnei. S. flexneri and S. sonnei were most resistant to ampicillin and co-trimoxazole. S. flexneri was more resistant to ampicillin than S. sonnei (p-value < 0.0001). Other reports from all over the world also indicate high resistance rates (58.3% to 85%) for ampicillin. Studies from Greece1² and Oregon1³ did not report species specific sensitivity, and results from Israel⁷ have shown similar high resistance rates for both S. sonnei and S. flexneri (87% and 71%, respectively).2,12-1⁴ The high rate of resistant to ampicillin and co-trimoxazole, which was reported in this study, was similar to those reported by the Centers for Disease Control and Prevention, NARMS data from 2001 and other studies.5,1¹ In Malaysia, the co-trimoxazole resistance rate of S. sonnei was relatively low (37.1%).¹ In one study, S. flexneri was much more resistant to chloramphenicol compared with S. sonnei. ¹⁴ This is also in accordance with the other reports. One study from Oregon reported a rate of 72% resistance of S. flexneri to chloramphenicol, whereas the resistance of S. sonnei was only 1%.1³ In our study, all Shigella species were sensitive to chloramphenicol, except S. flexneri, which was resistant to it (68%). Ampicillin and chloramphenicol were the inexpensive and broad-spectrum antimicrobial agents commonly used in developing countries. Increase of resistance to ampicillin or chloramphenicol is often related with extensive use of these antimicrobial agents.¹

This information is important because S. flexneri and S. sonnei are the most common species and it should be taken into account for treatment. Co-Trimoxazole is a common drug used as an empiric therapy in treatment of shigellosis and other diarrheal diseases of bacterial origins. The extensive use of this drug has contributed to the emergence of resistant S. sonnei and sustained the resistant trait.1,15,1⁶

In addition to co-Trimoxazole and ampicillin, ongoing resistance of Shigella species to tetracycline, chloramphenicol, and most recently to fluoroquinolones has been shown. Oral aminoglycosides and first and second generation cephalosporins are clinically ineffective, despite their in vitro activity.5,17-2¹ In our study Shigella species were sensitive to aminoglycosides, except kanamycin. Kanamycin is a member of aminoglycosides, which resistance to it was reported before by Lee et al. (2001) and Jeong et al. (2003).22,2³ They had reported the emergence of kanamycin-resistant S. sonnei isolates in Korea.¹ Although resistance to fluoroquinolones has been rarely reported, nearly all Shigella isolates are susceptible to these agents. Indeed, quinolones, which are also efficacious against other causes of bacterial gastroenteritis, are often recommended as empirical therapy in areas with high resistance to Shigella spp. They are, however, not approved for children because of the potential risk of damage to growing cartilage.2,6,2⁴ Another suitable antibiotic for treatment in children with severe shigellosis, especially in those who are hospitalized, is parenteral ceftriaxone, which is effective and usually recommended.² In this study, Shigella species (esp. S. dysenteriae and S. boydii) had high sensitivity to ceftriaxone.

In milder cases in children, choosing the optimal oral therapy is more problematic and should be based on local epidemiological data. Nalidixic acid or extended spectrum cephalosporins are usually adequate.² In our study, Shigella species (esp. S. dysenteriae and S. boydii) were highly sensitive to third generation cephalosporins (ceftiaxone, ceftizoxime, ceftazidime). Despite some studies that has reported resistance to nalidixic acid (esp. about S. flexneri and S. dysenteriae),8,9,2⁵ Shigella species were also sensitive to nalidixic acid, and S. flexneri isolates were even more sensitive to it than other species. This data makes nalidixic acid still one of the most appropriate antibiotics for treatment of shigellosis due to Shigella species' sensitivity to it, its cost and its easy accessibility in Iran. S. dysenteriae causes one of the most severe forms of epidemic severe dysenteries.2⁶ It was most sensitive to ceftriaxone, ceftizoxime, cephalotine, and co-trimoxazole; and most resistant to kanamycin, amikacin and nalidixic acid.

Multiresistance to the antimicrobial agents used in treatment of shigellosis has been reported in many parts of the world.1,22,23,27,2⁸ In our study, S. flexneri was more multiresistant than other species, which is very important due to its frequency. Ceftriaxone and ciprofloxacin have been shown to be highly effective for shigellosis.5,29-3¹ Ceftriaxone requires parenteral administration, whereas ciprofloxacin is avoided in children.

The increasing relative prevalence of Shigella species and the emergence of new resistant strains pose a clear public health problem. As resistance to antimicrobial agents changes constantly, it is important to keep these strains under surveillance in order to monitor the local susceptibility and subsequently formulate policies for the rational use of antimicrobial agents.

Submitted on: 05/23/2009

Approved on: 08/18/2009

We declare no conflict of interest.

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Publication Dates

• Publication in this collection
  11 June 2010
• Date of issue
  Apr 2010

History

• Received
  23 May 2009
• Accepted
  18 Aug 2009