Flow Microcalorimetric Measurements of the Antibacterial Activity of the Homologous Series m-Alkoxyphenols and p-Hydroxybenzoates on Escherichia coli

A ati vi da de an ti bac te ri a na dos mem bros de duas sé ri es ho mó lo gas (m-al co xi fe nóis e p-hi dro xi ben zo a tos de al qui la) foi es tu da da in vi tro so bre a res pi ra ção da Esche ri chia coli usan do mi cro ca lo ri me tria de flu xo. Os re sul ta dos para am bas as sé ri es mos tra ram uma re la ção li ne ar en tre log(dose) e a res pos ta ca lo ri mé tri ca (RC). Aná li se dos da dos per mi tiu a iden ti fi ca ção de con tri bu i ções de bi o a ti vi da de con cer nen te a es tru tu ra ma triz (a mo lé cu la me nos os gru pos n-CH2 pre sen tes na ca de ia la te ral) e a gru pos li po fí li cos, CH2.


Intro duc ti on
The m-al koxy phe nols and p-hydroxy ben zo a tes are recog ni zed an ti bac te ri als.The alkyl p-hydroxy ben zo a tes (pa ra bens) are wi dely used as pre ser va ti ves in cos me tic for mu la ti ons, phar ma ce u ti cals and fo ods 1 .The an ti bac te rial ac ti vi ti es of the m-al koxy phe nols have been de ter mi ned aga inst both Sal mo nel la typho sa (Gram-negative) and Myco bac te ri um pyo ge nes var.au re us (Gram-positive) 2 .The ac ti vity was shown to de pend upon the test or ga nism used.The ac ti vity aga inst Gram-negative bac te ria in cre ased as the hydrop ho bic cha in length in cre a sed.The `Hansch' cor re la ti on 3 pro po ses that bi o lo gi cal ac ti vity is de ter mi ned by li pop hi li city and by the so lu bi lity li mits in aque ous com part ments.Phe nols are cyto plas mic po isons 4,5,6 and hen ce the ir li pid so lu bi li ti es will de ter mi ne, to a lar ge ex tent, the ir abi lity to cross the cell mem bra ne.It has also been de mons tra ted that cell walls and cell mem bra nes are da ma ged by phe nols 7,8,9 .
As an al ter na ti ve to the clas si cal met hods for de ter mina ti on of a drugs bi o lo gi cal ac ti vity, mi cro ca lo ri metry has been pro po sed as a ge ne ral, non-destructive, high sen si tivity tech ni que which is in prin ci ple, ca pa ble of in ves ti gating any pro cess or re ac ti on 10 .This ge ne ra li za ti on, which in some ca ses may be per ce i ved as a li mi ta ti on (sin ce all che mi cal and physi cal pro ces ses give rise to chan ges in ent halpy), may also be seen as an ad van ta ge.This is be ca use it may be use ful to have ava i la ble a tech ni que which, for com plex he te ro ge ne ous systems, is uni ver sally ap pli ca ble.Mo re o ver as all bi o lo gi cal systems have a "bu ilt-in" spe cifi city con fer red upon them through the enz yma tic pro cesses which col lec ti vely sum to the ove rall me ta bo lism, then the ap pli ca ti on of mi cro ca lo ri metry may of fer in sights into me ta bo lism through the ob ser va ti on of the ther mal con sequen ces of that me ta bo lism.
The isot her mal po wer-leak flow mi cro ca lo ri me ter shown in Fig. 1 is most com monly used for the se stu di es.It con ta ins two mi xing ves sels or two flow-through ves sels.When the la ter is used, the re ac ti on mix tu re, e.g. , mi cro or -Ar ti cle ga nism plus drug is, pum ped through it by a pe ris tal tic pump from a re ac ti on ves sel out si de the ca lo ri me ter, and re tur ned to the re ac ti on ves sel, to form a clo sed loop system.The ins tru ment me a su res the po wer ge ne ra ti on by the re ac ti on pro cess wit hin the flow-through ves sel, re la tive to po wer chan ges in the empty flow-through ves sel.The po wer out put is am pli fi ed and re cor ded as a ca lo ri me ter res pon se or po tency p-t cur ve The out put of the mi cro ca lo ri me ter (dq/dt vs. t; a res ponse-time cur ve) pro vi des a con ti nu ous re cord of the pro gress and pro cess of me ta bo lism which, in con trast to many clas sical pro ce du res, res ponds to the me ta bo lic ac ti vity of only the ac ti ve mi cro bes pre sent and thus dis plays de ta ils of the dynamic and com plex pro cess of me ta bo lism.Bi o lo gi cal mi cro calo ri metry, has been used as a mo dern met hod for the de ter mi na ti on of bi o lo gi cal res pon se, in subs ti tu i ton for the clas si cal met hods such as tur bi di metry and res pi ro metry 11,12 .
The ad van ta ges of flow mi cro ca lo ri me ters for cell-drug stu di es are: 1. Be ca u se of its sen si ti vity and the fact that the ef fect of drug(s) on the who le me ta bo lism of the or ga nism may be exa mi ned, mi cro ca lo ri metry may re ve al more and also new de ta il of mi cro or ga nism-drug in te rac ti ons than ex -tand met hods can, es pe ci ally tho se which use a spe ci fic re acti on pro perly for analy sis; 2. The ca lo ri me tric met hod re quires only an ob ser va ble dif fe ren ce bet we en the ca lo ri me tric res pon se (CR) in the tre a ted and un tre a ted (con trol) in cu bati ons, and un li ke many ot her pro ce du res does not re qui re a trans pa rent so lu ti on; 3. The system can be au to ma ted.This is of par ti cu lar re le van ce for in dus tri al and cli ni cal ap pli ca tions; 4. The ac ti on of an ti mi cro bi al drugs can be stu di ed in real time con ve ni ently by me ans of the res pon se-time cur ve; 5.The me cha nism of the drug ac ti on is im ma te ri al: (both anti bac te ri al and an ti fun gal drugs can be exa mi ned); 6.The flow system is more akin to the in vi tro sta te than are, for exam ple, the systems used in dif fu si on as says; 7. The flow systems re pre sent the ef fect of drugs on the to tal me ta bo lism of the mi cro or ga nisms.This aga in more clo sely ap pro xi mates the in vi tro sta tes than do tech ni ques which me a su re a sin gle pa ra me ter of cell me ta bo lism for analy ti cal pur po ses.
The di sad van ta ges of the flow mi cro ca lo ri me ters in clude the fol lo wing: 1. Mi cro ca lo ri me ters are not suf fi ci ently sen si ti ve to me a su re the res pon se out put from cell con centra ti ons in the ran ge wit hin 10 2 -10 3 mL -1 .The pre sent detec ti on li mit is ca. 10 4 -10 5 cells mL -1 ; 2. Care has to be ta ken to avo id the pre sen ce of gas bub bles or the clum ping of cells in the flow li nes and mi cro ca lo ri me tric ves sels; 3. Some dif fi cul ti es, es pe ci ally with res pi ring cul tu res with high cell den sity, ari se be ca u se of the time ta ken for transfer from the re ac ti on ves sels to ca lo ri me ter, du ring which a subs tra te of the me di um, of ten oxy gen, can have its concen tra ti on chan ged suf fi ci ently in the me ta bo lic con di ti ons in the ca lo ri me ter to be dif fe rent from tho se in the re ac ti on ves sel.This ca pa city of the flow mi cro ca lo ri me ter to follow the de ta ils of me ta bo lism has, na tu rally, been ex ten ded to mi cro or ga nism iden ti fi ca ti on 10 , to drug-cell in te rac tions 13 and bi o as say 14 , and, in rat her less de ta il, for the stu dies of mo des of ac ti on of drugs upon sen si ti ve mi cro bes 10 .
Any subs tan ce that can mo dify the me ta bo lic pro cess in vol ved in cell growth and/or res pi ra ti on will, of cour se, chan ge the res pon se-t cur ve ob ta i ned from the ca lo ri me ter and so it be co mes pos si ble to study by a di rect met hod, the in te rac ti on bet we en me ta bo lic mo di fi ers and bi o lo gi cal cells in both a qua li ta ti ve and a quan ti ta ti ve way.The quan ti ta ti ve in ves ti ga ti on of drug bi o as say re sults in conven ti o nal li ne ar log(dose)-res pon se li nes.
In this work, we stu di ed the in vi tro an ti bac te ri al ac tivity of the ho mo lo gous se ri es of m-al koxy phe nols and p-hydroxy ben zo a tes by the res pi ra ti on of Esche ri chia coli , using a flow mi cro ca lo ri me tric tech ni que.It was also in tended to ve rify if CR can be par ti ti o ned into con tri bu ti ons of the li pop hi lic and hydrop hi lic parts of the mo le cu le (CH 2 groups and pa rent struc tu re, res pec ti vely).The de sig na ti on of li pop hi lic and hydrop hi lic su bu nits are iden ti cal to tho se used in the ther mody na mic analy sis of the par ti ti on of the se com pounds bet we en wa ter and non-aqueous sol vents using the Col lan der equa ti on 15 .

Expe ri men tal
The met hoxy and et hoxy de ri va ti ves of re sor ci nol were pur cha sed from East man Ko dak Co. Ltd. and were re pe atedly dis til led un der va cu um be fo re use.The ot her re sor ci -  The the o re ti cal amount of a 25% aque ous po tas si um hydro xi de so lu ti on was ad ded drop by drop du ring 3 h follo wed by bo i ling un der re flux for a furt her 2 h.Small quanti ti es of di et hers were for med in the se re ac ti ons, par ti cu larly with the hig her mem bers of the se ri es for which pro lon ged re ac ti on ti mes were ne ces sary.The re acti on mix tu re was al lo wed to cool, pou red onto ice, aci di fi ed and ex trac ted with et her.Excess re sor ci nol was re mo ved by re pe a ted was hing with wa ter and the mo no alkyl et hers ex trac ted into po tas si um hydroxy de so lu ti on.Aci di fi ca ti on of the al ka li ne so lu ti on fol lo wed by et her ex trac ti on yi elded the alkyl et hers which were dis til led se ve ral ti mes under re du ced pres su re (1.5 mmHg).All the mo no et hers were as sa yed for pu rity by GLC, on a Per kin-Elmer F33 Gas Chro ma to graph fit ted with a 1m co lumn con ta i ning 3% SE 30 on Chro mo sorb W, 80-100 mesh (oven tem pe ra tu re 110 °C; in jec tor/de tec tor tem pe ratu re 150 °C; car ri er gas N 2 (30 mL min -1 ), mix tu re of hydro gen (30 mL min -1 ) and air (300 mL min -1 ) used for fla me io ni za ti on de tec tor).NMR spec tros copy was used to con firm the struc tu res of the com pounds and to de tect impu ri ti es, if pre sent.
p-Hydroxy ben zo a tes were the ge ne rous gift of NIPA che mi cals and were cer ti fi ed as 99.9% pure on de li very.They were used wit hout furt her tre at ment.
Esche ri chia coli NCTC 10418 was grown in 250 mL flasks con ta i ning 40 mL of me di um of com po si ti on (g L - O, 0.0003; fi nal pH 7.0.Forty flasks were ino cu la ted with 0.8 mL of an over night cul tu re and in cu bated at 37 °C on a ro tary sha ker (220 rpm: Gal len kamp, U.K.).Growth was fol lo wed by op ti cal den sity (EEL co lori me ter) and 6 h af ter in cu ba ti on, at an op ti cal den sity equiva lent to 1.2 g dry we ight of cells/li tre, the cells were po o led, cen tri fu ged, was hed twi ce in 1/4 strength Rin ger's so lu ti on con ta i ning 10% w/w of di methylsul fo xi de (DMSO) at 4 g dry we ight of cells L -1 .After the last cen trifu ga ti on, the cells were re sus pen ded in Rin ger's so lu ti on con ta i ning 10% of DMSO and con di ti o ned in 2 mL polypropy le ne am pou les.The am pou les were in ser ted into a thin per fu ra ted Styro fo am pla te which was the lid of an alumi num con ta i ner for li quid ni tro gen.The Styro fo am pla te was man ta i ned at a he ight of 8 cm abo ve the ni tro gen le vel and when the tem pe ra tu re in the con trol am pou le re a ched -80 °C (me a su red with an al co hol ther mo me ter), the ampou les were li be ra ted for im mer si on in li quid ni tro gen and sto red in a cryo ge nic cylin der 16 .The vi a ble count was done pe ri o di cally, gi ving 1.3 x 10 10 cells mL -1 .Cells sto red for 6 months were re co ve red with 95% vi a bi lity.
The an ti bac te ri al ac ti vity of the com pounds of the two ho mo lo gous se ri es were de ter mi ned by using an isot her mal dif fe ren ti al mi cro ca lo ri me ter (LKB-10700) at 36 o C, fit ted out with a flow-through cell.The mi cro ca lo ri me ter sche me is shown in Fig. 1.The me dia is pum ped into the flow-through cell (35 mL h -1 ) with the help of a LKB-2132 pe ris tal tic pump.The heat out put from the res pi ra ti on me ta bo lism of E. coli is de tec ted by the pair of ther mo pi les con nec ted around the flow-through cell 17 .The com po si ti on of the cul tu re media used for the res pi ra ti on of E. coli was (g L -1 ): glu co se, 0.5; K 2 HPO 4 , 8.75; K 2 PO 4 , 3.75; pH = 7. Wit hout the ad diti on of the bi o ac ti ve com pounds, this me dia was de fi ned as a con trol pre pa ra ti on.The ino cu la ti on of mi cro or ga nisms (500 µL of the cell sus pen si on) was al ways done im me di ately af ter tha wing the am pou le for 3 min in a wa ter bath at 36 °C, fol lo wed by agi ta ti on of 20 s.

Re sults and Dis cus si on
Fi gu re 2 shows the ther mo grams for the res pi ra ti on of E. coli .The con trol cur ve is the ther mo gram of E .coli in Ta ble 2. Va lu es of CR, log(dose), log(dose) max and li ne ar le ast squa re analy sis data for the in te rac ti on of p-hydroxy ben zo a tes with E. coli .this pro cess, wit hout the ad di ti on of any drug, ob ta i ned by pum ping the ae ra ted bac te ri al sus pen si on through the flowthrough cell by me ans of a pe ris tal tic pump, Fig. 1.

Com pounds
The ot her cur ves of Fig. 2 show the ef fect of the in cre asing con cen tra ti on of m-pro poxy phe nol on the res pi ra ti on pro cess.In bi o lo gi cal mi cro ca lo ri metry, the ca lo ri me tric res pon se, CR, is de fi ned com pa ring the ma xi mum he ight of the ther mo gram in each con cen tra ti on of the drug with the ma xi mum he ight of the con trol cur ve (100% CR) 10 .Also, the con cen tra ti on of the drug is de fi ned as dose and the value of log (dose) when CR = 0 is de no mi na ted log (dose) max .It is ob ta i ned by ex tra po la ti on of the cur ves CR vs. log (dose) and re pre sents the lo ga rithm of the concen tra ti on of the drug which if ad ded to the cell me dia should not pro du ce a res pon se in the ca lo ri me ter.In this work the va lu es of log (dose) max for each com pound were ob ta i ned by ex tra po la ti on from Figs. 3 and 4.
From the mi cro ca lo ri me tric study of the ef fect of the bio ac ti ve com pounds, m-al koxy phe nols and p-hydroxy benzo a tes, on the res pi ra ti on of E. coli , it was pos si ble to de mons tra te the exis ten ce of the li ne ar cor re la ti on bet we en log (dose) and the ca lo ri me ter res pon se.Fi gu res 3 and 4 show the ca lo ri me tric data ob ta i ned from the ther mo grams of in te rac ti on of the ho mo lo gous se ri es of m-al koxy phenols and p-hydroxy ben zo a tes with E. coli cells, using the CR va lue cal cu la ted as was shown in Fig. 2.
Ta bles 1 and 2 show the ca lo ri me tric res pon se data of the com pounds of the two ho mo lo gous se ri es to get her with the cor re la ti on co ef fi ci ent, ob ta i ned from Figs. 3 and 4.
A graph of log(dose) max vs. num ber of -CH 2 groups present in the si de-chain gave a cor re la ti on co ef fi ci ent of -0.9931 and -0.9877, in ter cepts of 5.428 and 4.671 and slopes of -0.484 and -0.339 for the ho mo lo gous se ri es of m-alkoxy phe nols and p-hydroxy ben zo a tes, res pec ti vely, Fig. 5.
For each ho mo lo gous se ri es of the com pounds stu di ed here, we de fi ned a "pa rent" struc tu re, m-alkoxyphenols (I) and p-hydroxybenzoates (II) (Sche me 1).
The va lue of the in ter cepts in con cen tra ti on terms, 268,000 µM and 47,000 µM are then the con cen tra ti ons (hypot he ti cal) of the pa rent struc tu re for the se ri es m-alkoxy phe nol and p-hydroxy ben zo a te, res pec ti vely.
It is no ta ble from Fig. 5, but per haps not sur pri sing, that the pa rent struc tu re of the phe nol is less bi o ac ti ve than is the pa rent struc tu re of the p-hydroxy ben zo a te.Mo re o ver, the con tri bu ti on per CH 2 group to the va ri a ti on in bi o ac ti vity is dif fe rent for the two ho mo lo gous se ri es but, as the hydropho bi city of the se com pounds in cre a ses, the bi o ac ti vity gets clo ser, be ing prac ti cally the same for the five car bon cha ins.
The va lue of log (dose) max al lows the eva lu a ti on of the re la ti ve an ti bac te ri al ac ti vity of the com pounds, sin ce all the bi o ac ti vi ti es are re la ted to the same re pro duc ti ve pattern, that is, com ple te inhi bi ti on of heat pro duc ti on in the mi cro ca lo ri me ter (CR = 0).For the stu di ed an ti bac te ri al com pounds the ad di ti vity prin ci ple was ob ser ved as well as that CR can be par ti ti o ned into a bi o ac ti vity con tri bu ti on of the pa rent struc tu re plus the num ber of CH 2 groups pre sent in the side cha in.
The CR data ob ta i ned through flow mi cro ca lo ri metry shows that the re is gre at po ten ti al for mi cro ca lo ri me tric me a su re ments in ba sic sci en ces, me di ci ne, phar ma co logy and to xi co logy.The fu tu re for ca lo ri metry in this fi eld is bright, now that the tech ni ques have been es ta blis hed and, with the de ve lop ment of im pro ved ins tru men ta ti on for moni to ring ther mal and ot her sup ple men tary pro per ti es of systems, it will be pos si ble to make even more de ta i led studi es of cel lu lar systems.

2 Fi gu re 1 .Fi gu re 2 .
Di a gram of the isot her mal dif fe ren ti al mi cro ca lo ri me ter.Ca lo ri me ter res pon se-t cur ves ob ta i ned for res pi ra ti on of E. coli .Con trol cur ve re pre sents the res pon se of bac te ri al cells alo ne in glu co se buf fer.Other cur ves re pre sents the same pro cess with dif ferent con cen tra ti ons of m-pro poxy phe nol.Fi gu re 3. CR vs. Log (dose) for m-al koxy phe nols.
Com pounds: N = 1 m-met hoxy phe nol N = 2 m-et hoxy phe nol N = 3 m-pro poxy phe nol N = 4 m-bu toxy phe nol N = 5 m-pen toxy phe nol nol mo no alkyl et hers were pre pa red by dis sol ving equi mo lar amounts of re sor ci nol and alkyl bro mi de in the ap pro pri a te al co hol fol lo wed by gen tle bo i ling un der reflux.

Fi gu re 5 .Fi gu re 4 .
Effect of CH 2 groups on log (dose) max va lu es for m-al koxyphe nols and p-hydroxy ben zo a tes.CR vs. log (dose) for p-hydroxy ben zo a tes.
Com pounds: N = 1 methyl p-hydroxy ben zo a te N = 2 ethyl p-hydroxy ben zo a te N = 3 1-propyl p-hydroxy ben zo a te N = 4 1-butyl p-hydroxy ben zo a te N = 5 1-pentyl p-hydroxy ben zo a te Ta ble 1. Va lu es of CR, log(dose), log(dose) max and li ne ar le ast squa re analy sis data for the in te rac ti on of m-al koxy phe nols with E. coli .