Group 1 beta-lactamases of Aeromonas caviae and their resistance to beta-lactam antibiotics.

The contribution of beta-lactamase production to beta-lactam antibiotic resistance was examined in an Aeromonas caviae mutant strain, selected in vitro by cefotaxime and derived from a wild-type strain isolated in our laboratory from crude sewage. Both strains produced beta-lactamase. The mutant strain (AC7m) produced beta-lactamase constitutively, in contrast to the parental strain (AC7), which was inducible by cefoxitin. AC7m was regarded as a mutant from AC7, which over-expressed beta-lactamase. The mutant strain showed a remarkable reduction in sensitivity to most of the beta-lactam antibiotics tested, such as (i) aminopenicillins and their combinations with clavulanic acid and sulbactam, (ii) carboxypenicillins, (iii) ureidopenicillins, and (iv) cephalosporins. This strain remained susceptible to ceftazidime, imipenem, and aztreonam. Isoelectric focusing of sonic extracts revealed that both strains AC7 and AC7m shared a common major beta-lactamase band at pI 6.5. The plasmid DNA assays showed that the beta-lactamases expressed by each A. caviae strain were chromosomally encoded. Based on substrate and inhibitor profiles determined in sonic extracts for AC7 and AC7m, the enzymes displayed on isoelectric focusing at pI 6.5 were assigned to chromosomal Group 1 beta-lactamases. Imipenem would therefore be the appropriate choice for therapy of infections caused by A. caviae beta-lactamase over-expressing mutants.

Identification and antibiotic resistance of faecal enterococci isolated from water samples.

We have isolated 47 strains of presumptive faecal streptococci from different water samples. Identification was made by the method of Facklam et al. (1989). Antibiotic resistance was studied on Mueller-Hinton Agar. Twelve antibiotics were tested. High-level aminoglycoside resistance (HLAR) and resistance to glycopeptides were studied. Biochemical identification of presumptive faecal streptococci isolates gave the following results: 19 Enterococcus faecalis, 12 E. faecium, 8 E. hirae, 4 E. durans and 4 E. mundtii. E. mundtii is not included among faecal enterococci. None of the strains were resistant to glycopeptides (vancomycin and teicoplanin). Three strains of Enterococci showed HLAR. Two of them were isolated from coastal bathing waters and the other from wastewater. This suggests that water could contribute to spread of HLAR enterococci and it should be a matter of concern for public health authorities.

Antibacterial susceptibility patterns of Pseudomonas strains isolated from chronic canine otitis externa.

Development of antibiotic resistance in bacteria is a problem of great concern. It is important to establish the convenience of antimicrobial susceptibility tests in animal infections. The aim of this study was to test the susceptibility to antibiotics of Pseudomonas strains isolated from chronic canine otitis externa. We tested 23 strains of Pseudomonas: 19 Ps. aeruginosa, three Ps. fluorescens and one Pseudomonas spp. The most effective antibiotics were tobramycin (100% susceptible), marbofloxacin (91.3%) and ceftazidime (91.3%). Ticarcillin and gentamicin, commonly used for the treatment of otitis externa also showed good results (susceptibility of strains was 86 and 65.2% respectively). Lower susceptibility was found using enrofloxacin (52.1%) probably due to its indiscriminate use. We emphasize the need for a rational policy of antibiotic prescribing in order to prevent the selection of resistant strains.