beta-Lactam resistance of motile Aeromonas isolates from clinical and environmental sources.

The MICs of various beta-lactams for 182 isolates of Aeromonas species, i.e., A. hydrophila (n = 101), A. sobria (n = 69), and A. caviae (n = 12), from clinical and environmental sources were determined by an agar dilution technique. All strains were resistant to ampicillin and susceptible to aztreonam. A. sobria and A. caviae demonstrated lower resistance rates than A. hydrophila. Penicillin-hydrolyzing beta-lactamases were detected in all strains.

Species of motile Aeromonas, an inhabitant of water environments, are increasingly being reported as important pathogens causing gastroenteritis and severe extraintestinal diseases, such as septicemia and peritonitis, in immunocompromised hosts as well as healthy individuals (2,4,5,7,8). The role of antibiotic therapy in the management ofAeromonas infections has not yet been defined (14,15), although the antibiotic resistance in motile Aeromonas spp. is an important problem for therapy directed to these organisms. Most clinical isolates of motile Aeromonas spp. resistant to 3-lactams are resistant to penicillins (16). However, some interspecies differences in susceptibility were observed with cephalosporins (9,15).
New P-lactams that possess greater activity against gramnegative and gram-positive bacteria have been developed. In particular, penems and carbapenems such as imipenem inhibit a broad range of microorganisms, including motile Aeromonas spp. (3,11). However, some investigators recently reported the appearance of imipenem-resistant isolates of Aeromonas spp. caused by inducible ,B-lactamases that are active against carbapenems (1,6,16). The present study examined the speciesassociated r-lactam susceptibility patterns and ,B-lactamase production of clinical and environmental isolates of A. hydrophila, A. sobria, and A. caviae.
Organisms. A total of 182 Aeromonas isolates consisting of 109 strains from the feces of patients with diarrhea acquired in Japan, Southeast Asia, and the People's Republic of China and 73 strains from environmental sources such as food, fresh water, and seawater collected in the Tokyo metropolitan and Kanagawa Prefecture areas were examined in the present study.
The species of the Aeromonas isolates were determined by the criteria of Popoff (13) and Janda et al. (8).
Susceptibility testing. MICs were determined by an agar dilution technique with sensitivity disk agar (Eiken Chemical Co.) containing graded concentrations of antibiotics and an inoculum of approximately 104 cells per spot, which was applied with a multi-inoculator. Plates were incubated at 35°C for 18 h. The following standard antibiotic powders were tested: ampicillin and imipenem (Banyu Pharmaceutical Co. I-Lactamase testing. 3-Lactamase production was determined by the benzylpenicillin substrate method with benzylpenicillin disks (Beta-Lactamase Detection Paper; Oxoid, Basingstoke, United Kingdom) and was also determined by the chromogenic cephalosporin substrate method (12) with nitrocefin disks (Cefinase disk; BBL Microbiology Systems, Cockeysville, Md.).
The results of the susceptibility tests are shown in Table 1. All strains were uniformly resistant to ampicillin and were susceptible to aztreonam. Piperacillin showed variable activity against isolates of each species and was more active than the other penicillins tested. The ticarcillin-clavulanate combination was active, and the MIC of this combination for 90% of the strains was fourfold lower than that of ticarcillin alone. Of three species tested, the MICs of cephalosporins, monobactam, and carbapenem for 90% of A. hydrophila isolates tested were two-to eightfold higher than those for A. sobria and A. caviae isolates. Eight of the 101 A. hydrophila strains (8%) and 2 of the 69 A. sobria strains (3%) were resistant to imipenem. No differences in susceptibility were observed between environmental and clinical isolates ofA. hydrophila, A. sobria, and A. caviae (data not shown).
Results of analysis of ,B-lactam cross-resistance are given in Table 2. Three A. hydrophila strains resistant to cefotaxime, ceftriaxone, and moxalactam were uniformly resistant to the other cephalosporins tested. Twenty-five percent or more of imipenem-resistant A. hydrophila strains were resistant to cefotaxime, ceftriaxone, cefoperazone, and moxalactam, while the two imipenem-resistant strains of A. sobria were susceptible to these four cephalosporins.
These differences in the resistance patterns against newer 3-lactams suggest that imipenem resistance in A. hydrophila and A. sobria is associated with some distinct 3-lactamase activities (16). In our results of ,B-lactamase testing, penicillinhydrolyzing ,-lactamase production was observed in all strains of A. hydrophila, A. sobria, and A. caviae. Recently, we found and analyzed some inducible or stably derepressed imipenemhydrolyzing ,B-lactamases in imipenem-resistant isolates of A. hydrophila and A. sobria used in the present study (data not shown and unpublished data).  Resistance to various antibiotics has previously been observed, especially in A. hydrophila isolates in comparison with other species of Aeromonas (9). This tendency of ,B-lactam resistance was also observed in our study.
These data suggest that the species-associated 1-lactam resistance of motile Aeromonas spp. has important implications in the selection of definitive species-oriented therapy of infectious diseases caused by motile Aeromonas spp.
Our results demonstrate that aztreonam has good in vitro activity against motile Aeromonas spp.