554

Correspondence fragilis NCTC 9343. Bonafide resistant strains should be referred to the Anaerobe Reference Unit for further study. J. S. BRAZIER VALERIE HALL B. I. DUERDEN PHLS Anaerobe Reference Unit, University Hospital of Wales. Heath Park. Cardiff CF4 4XW. UK References Cederbrant, G., Kahlmeter, G. & Ljungb, A. (1992). Proposed mechanism for metronidazole resistance in Helicobacter pylori. Journal of Antimicrobial Chemotherapy 29, 115-20. Edwards, D. I. (1979). Mechanism of antimicrobial action of metronidazole. Journal of Antimicrobial Chemotherapy 5, 499-502. Sebald, M., Reysset, G. & Breuil, J. (1990). What's new in 5-nitroimidazole resistance in the Bacteroides fragilis group? In Clinical and Molecular Aspects of Anaerobes (Bordello, S. P., Ed.), pp. 217-25. Wrightson Biomedkal Publications. The activity of ceflxinte against 715 urinary isolates of Enterobacteriaceae isolated from general practice and out-patients in twenty centres across the British Isles J Antimicrob Chemother 1992; 30: 554-556 Sir, Cefixime is an orally available cephem structurally related to some third generation cephalosporins and showing similar antibacterial activity (Kamimura el al., 1984; Bauernfeind, 1985). It is stable to a wide range of chromosomal and plasmid coded /?-lactamases and, hence, is active against Enterobacteriaceae resistant to ampitillin, cephalexin and cefaclor (Sanders, 1989). Therefore cefixime is likely to be of value in the therapy of some urinary tract infections in an out-patient setting. The objective of our study was to establish the in-vitro activity of cefixime against more than 700 Gram-negative urinary pathogens isolated in twenty centres across the United Kingdom with an emphasis on isolates from general practice and out-patients. Clinical laboratories were asked to collect consecutive and clinically significant nonrepeat isolates from general practice and outpatients. Each laboratory contributed 10 Escherichia coli and 50 other urinary tract pathogens (excluding Pseudomonas spp. and Enterococcus faecalis). A total of 715 strains were collected and sent to Southmead Hospital, Bristol for testing. MICs of cefixime,

Downloaded from http://jac.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 10, 2015

years, resistance to this agent is still unusual, although plasmid-mcdiatcd resistance to the nitroimidazoles has been reported (Sebald, Reysset & Breuil, 1990). Recently an upward trend in the number of referrals of putative resistance to mctronidazolc among the Bacteroides fragilis group became apparent. In the space of less than one month, five supposedly metronidazole resistant strains of B. fragilis were submitted to the Unit for confirmation. Four laboratories, (two isolates were from one laboratory) reported isolates which showed greatly reduced sensitivity to a 5 fig metronidazole disc. The strains were isolates from a hip abscess, a blood culture, a wound swab (site unspecified), and two from high vaginal swabs. Upon subculture in the Anaerobe Reference Unit under controlled anaerobic conditions, all five strains proved to be fully susceptible on disc testing to a 5 m metronidazole disc. Four of the strains were identified as B. fragilis, (sensu stricto) and one as B. distasonis. Further examination using the E Test (AB Biodisk Sweden) showed MICs of metronidazole ranging from 0-38 to O75mg/L—well within therapeutic levels. The most probable explanation for this apparent resistance found by the submitting laboratories was sub-optimal anaerobiosis. The 'B. fragilis group' are very aero tolerant and will grow in the presence of small amounts of oxygen. Metronidazole only becomes active when it is reduced via its nitro group accepting electrons from a low-redox-potential electron transport enzyme pyruvate:ferredoxin oxidoreductase (Edwards, 1979). If conditions are not sufficiently reduced ( < 430 mV) the drug will not become reduced to its active form. The end result is bacterial growth with either no zone or a very reduced zone to the metronidazole disc. Some credence was given to this explanation in a telephone conversation with the laboratory that submitted two 'resistant' isolates, who reported having reliability problems with their anaerobic chamber. As the submitting laboratories were widespread in their geographical location (London and the north of England) it is unlikely that a single factor (i.e. a common batch of faulty anaerobic gas mixture or metronidazole discs) contributed to these reports of false resistance. It is more likely that a series of local factors leading to incomplete anaerobiosis was responsible. Diagnostic laboratories identifying possible resistance to metronidazole with the B. fragilis group are advised to check this result by using a control strain of known sensitivity such as B.

Correspondence

•SJ

a

B s:

:§:

us

A A A A A A A A A

P

A A

A A A A A

A A

A A

A A A A A A A A

A

A A

A A A A A A A A A o R A A

a

.is

— (N O —

6 6 6 oo «o — oooo

The activity of cefixime against 715 urinary isolates of Enterobacteriaceae isolated from general practice and out-patients in twenty centres across the British Isles.

554 Correspondence fragilis NCTC 9343. Bonafide resistant strains should be referred to the Anaerobe Reference Unit for further study. J. S. BRAZIER...
224KB Sizes 0 Downloads 0 Views