Comparison of antibiotic susceptibility of old and current Serratia

Aims: We explored changes in antibiotic susceptibility of Serratia marcescens in the last 50 years by comparing isolates collected between 1945 and 1950, and current isolates. Materials & methods: Isolates were divided into three groups: environmental, clinical and 'old'. Susceptibility was determined by microdilution. Class 1 integrons were determined by PCR. Statistical analysis was conducted using the Kruskal-Wallis (K-W) tests with Bonferroni correction for multiplicity. Antimicrobials showing differences in the K-W test were analyzed by Mann-Whitney U test. Differences were considered significant when p < 0.05. Results: All isolates were sensitive to ceftazidime, cefotaxime, kanamycin, gentamicin, ofloxacin and ciprofloxacin, and resistant to rifampicin, penicillin, ampicillin, amoxicillin, tetracycline, amoxicillin-clavulanic acid, cefazolin, cefamandole, polymyxin B/colistin, fusidic acid, lincosamides, streptogramins, daptomycin, linezolid and cefuroxime. Old isolates exhibited reduced susceptibility to streptomycin. Cefotaxime and streptomycin showed significant differences in the K-W test. None of the strains studied presented ESBL. Resistance to antimicrobials was not drastically different in Serratia when old and current strains were compared. Conclusion: Despite the multiple molecular mechanisms involved in bacterial resistance, withdrawing the antibiotics tends to restore the original phenotypes. Results from this report essentially confirm the conclusion obtained through metagenomic analysis that resistance to antibiotics already existed in ancient times.