Antibiotic Susceptibility of Clinical Isolates of <i>Serratia marcescen</i><i>s</i> Compared with Sensitivity to Group A (Phage Tail) Bacteriocins

Abstract

A total of 564 clinical isolates of <i>S. marcescens</i>, mostly from postoperative wound and soft tissue specimens and from the lower respiratory tract and the urinary tract of patients, were bacteriocin typed. Of these, 532 isolates (94%) proved typable. The isolates comprised 30 bacteriocm types; however, nearly half of the isolates constituted bacteriocin type 18. Next in frequency were bacteriocin types 1, 4, 9, 14, 15, 21, and 44. Amikacin inhibited the growth of 396 of 399 tested isolates (99.3%). Cotrimoxazole, nalidixic acid, and chloramphenicol were effective against 72.9, 75.4, and 61.7% of the 564 isolates, respectively. When the Bauer-Kirby disk antibiograms of isolates comprising the most commonly encountered bacteriocin types were analyzed, marked differences in susceptibilities to carbenicillin, chloramphenicol, aminoglycosides except amikacin, nalidixic acid, sulfonamides, and cotrimoxazole were noted. One strain of <i>S. marcescens</i> (bacteriocin type 44, serotype 06/014:H4), which had caused biliary tree infection in a surgical patient, yielded phenotypic variants, of which the ‘gray’ colony type proved resistant against amikacin and susceptible to nalidixic acid, and less resistant against polymyxin B, whereas variants of the ‘graywhite’ colony type proved susceptible to amikacin, less susceptible to nalidixic acid, and highly resistant against polymyxin B. Amikacin resistance of the ‘gray’ variants could by eliminated (‘cured’) with acridine orange, but was conjugally nontransferable. In contrast, the ‘graywhite’ variants of this strain transferred from 10 to 11 resistance markers to recipient strains of <i>S. marcescens</i> that had been subjected to mild heat treatment (50°C, 20 min) prior to cocultivation.