In vitro activities of antimicrobial agents against Proteus species from clinical specimens

Two hundred clinical isolates of members of the genus Proteus were definitively identified and their antimicrobial susceptibilities to 12 antimicrobials tested, 176 isolates (88%) being identified as Proteus mirabilis, 12 strains (6%) as Proteus vulgaris and 12 strains (6%) as Proteus penneri. Most strains were isolated from pus (62.5%) and urine (34%), but in general there were no significant differences in the rates of isolation of any of the species by age or sex, although it was noted that P. vulgaris was only isolated from patients belonging to the older age group (> 5 years). The Proteus spp. were notably susceptible to nalidixic acid, ceftazidime and the aminoglycosides tested, and resistant to polymyxin B and colistin. The inclination of certain Proteus species to be susceptible or resistant to certain antimicrobials was noted, but strain differences also existed. The results of the study confirm the importance of performing antimicrobials susceptibility testing of each Proteus isolate to avoid potentially misleading therapy. The noted discrepancy in the result of the susceptibility of P. penneri to chloramphenicol as tested by different standard methods merits further investigation.