Resistance to antibiotics and biocides among non-fermenting Gram-negative bacteria

Abstract

Objective: To investigate the antibiotic and biocide susceptibilities of clinical isolates of rarely encountered Gram-negative, non-fermenting bacteria.

Methods: Thirty Gram-negative non-fermenting bacterial strains were isolated from blood cultures of oncology patients. These were studied for their resistance to 11 antibiotics. Their susceptibilities to seven biocides used in hospitals were also examined.

Results: Isolates of Stenotrophomonas maltophilia and Ochrobactrum anthropi were generally resistant to at least five of the antibiotics, whereas isolates of Comamonas acidivorans, Flavobacterium oryzihabitans, Aeromonas hydrophila, Sphingobacterium spiritivorum, Acinetobacter junii and Acinetobacter lwoffi were generally sensitive to at least nine of the antibiotics. Trovafloxacin and trimethoprim-sulfamethoxazole were the most effective antibacterial agents tested, with 0% and 7%, respectively, of isolates being resistant, whereas 63% of isolates were resistant to aztreonam. Some isolates, sensitive to meropenem and/or ceftazidime in vitro, possessed very high MBC/MIC ratios for these beta-lactams. Two out of three biocides used in hospital pharmacies showed lethal activity towards all strains tested when used at less than one-third of their recommended in-use concentration. Proceine 40 failed to give a 5 log reduction in bacterial cell number for the isolates tested when used at its "in-use" concentration. A concentration of > 500 mg/L chlorhexidine was required to achieve a 5 log reduction for the same isolates.

Conclusions: We have examined the antibiotic susceptibilities of non-fermenting Gram-negative bacterial strains isolated from immunocompromised patients. Despite being sensitive to certain antibiotics in vitro, some isolates were still able to cause serious bacteremia. We have also reported for the first time the susceptibilities of non-fermenting Gram-negative bacteria to common biocides used in hospital infection control, and have shown that some strains are able to persist at the "in-use" concentration of particular biocides. It is therefore important to study further this particular group of organisms, and, in particular, to examine whether there exists a link between resistance to antibiotics and resistance to biocides.