Beta-lactam resistance profiles in urinary tract infection among Escherichia coli isolates in Bloemfontein

[摘要] English: This study was designed to elucidate the epidemiology, nature and extent of β-lactamresistance in urinary tract infections caused by Escherichia coli isolates in Bloemfonteinhospitals. To reach this goal it was necessary to phenotypically characterise and re-identifythe E. coli isolates by the Mastascan identification system. Pure cultures were obtained bystreaking single colonies onto MacConkey agar containing 50 µg / ml ampicillin. Singlecolonies were then picked off and inoculated into Mueller-Hinton broth, grown overnight at37°C and re-streaked onto MacConkey agar containing 50 µg / ml ampicillin. Three colonieswere then picked and stored at -20°C in a freeze mixture of 10 % proteose and 10 % glycerol.E. coli isolates were mated to a universal recipient strain (162) in pre-warmed Mueller-Hinton broth. Transconjugants were selected on MacConkey agar supplemented with 50 µg /ml ampicillin and 50 µg / ml nalidixic acid. Lactose-negative colonies resistant to nalidixicacid and ampicillin were inoculated into Mueller-Hinton broth, incubated for six hours and restreakedonto MacConkey agar containing ampicillin. Lactose-negative colonies were pickedand considered to be transconjugants. It was found that fifty four (45 %) out of 120ampicillin-resistant isolates transferred ampicillin-resistance determinants to an E. colirecipient (162) by conjugation. Seventy-five percent of ampicillin-resistant isolates were fromfemale patients, indicating that urinary tract infections are more prevalent in females than inmales.The National Committee for Clinical Laboratory Standards* agar dilution method wasused to determine minimum inhibitory concentration (MIC) distributions of 12 β-lactamantimicrobial agents (ampicillin, amoxycillin, piperacillin, augmentin, cefoxitin, cefotaxime,cefepime, ceftazidime, cephazolin, ceftriaxone, cefuroxime and imipenem). Strains found tobe resistant had MICs that overlapped the range where susceptibility was normally assumed.This was due to inducible β-lactamase producer strains, which go undetected by the Kirby-Bauer disk diffusion method but can be identified by using the Jarlier double-disk method.MIC frequency distributions for the penicillins showed that elevated doses should beadministered in order to maximise antibiotic concentration at the site of infection or that asecond antibiotic agent or inhibitor should be used in combination therapy.Beta-laetam susceptibility profiles were determined by the Kirby-Bauer disk diffusionmethod. This method was also used to determine the correlation of MIC values with theinhibition zone diameters in order to predict treatment outcomes from inhibition zonediameters.The Jarlier double-disk technique was used to detect extended-spectrum β-lactamaseproducingorganisms and the frequencies at which they occurred. There was no differencebetween the ratios of ESBL-producers in hospitalised and non-hospitalised patients, althoughthe absolute numbers were different. This was probably due to the 48 hour cut-off point usedto define hospitalisation. Samples taken before 48 hours were considered to be nonhospitalised.There were many more female than male patients with urinary tract infections inthe Bloemfontein hospitals during the period of the study.The extent of joint resistance to β-lactam antibiotics among E. coli isolates was assessedby comparing two agents at a time. The observed incidence of joint resistance was comparedto the rate of double resistance expected if it had been acquired as two independent events.It was found that even amongst two antibiotics that are biologically cross-resistant(ampicillin and augmentin) a close correlation exists between the concentrations of the twoagents required to inhibit individual E. coli strains.

[发布机构] University of the Free State