Antibiotic susceptibility of Serratia marcescens and Serratia liquefaciens

Background: Over a period of 20 years, a total of 1,603 Serratia isolates were recovered from clinical specimens and examined for susceptibility to 29 antimicrobial drugs using the Bauer-Kirby agar disk diffusion test. Serratia marcescens was recovered most frequently (n = 1,409), followed by S. liquefaciens (n = 172); other Serratia species were scarce. During the 2-decade observation period there occurred 35 putative episodes/clusters of nosocomial cross-infection and 1 pseudo-outbreak due to S. marcescens, but none due to S. liquefaciens. Methods: The antimicrobial susceptibility data for S. marcescens and S. liquefaciens were subdivided into two observation periods: I = 1980-1993, and II = 1993-1999. The crude data (series A) obtained for S. marcescens were corrected in two ways: by the omission of repetitive patient isolates (series B) and the additional removal of outbreak isolates except for index case isolates (series C). Results and Conclusions: Comparison of data obtained in series IC and IIC disclosed an increase in the susceptibility of S. marcescens to ampicillin + sulbactam, cefotaxime, chloramphenicol, doxycycline, fosfomycin, gentamicin, piperacillin, piperacillin + tazobactam, timentin and tobramycin during observation period II. Conversely, there was a decrease in susceptibility to ciprofloxacin, nalidixic acid and trovafloxacin, and slightly diminished susceptibility to norfloxacin and ofloxacin during observation period II as compared with the previous period. The crude data obtained for S. liquefaciens required no correction, as there were only a few repeat isolates. There was an increase in susceptibility to ampicillin, ampicillin + sulbactam, cefuroxime, doxycycline, fosfomycin, nitrofurantoin and polymyxin B (clear inhibition zones). However, there was an inexplicable decrease in susceptibility to piperacillin + tazobactam. Cocarde growth around polymyxin B disks was noted with 55.8% of the S. marcescens isolates as compared with 6.8% of the S. liquefaciens isolates. Slime around fluoroquinolone inhibition zones was produced by 83.4% of the S. marcescens isolates. Slime production around carbapenem inhibition zones was noted with 52% of the S. liquefaciens isolates, but with only a single isolate of S. marcescens. Copyright (C) 2000 S. Karger AG, Basel.