Aeromonas Species from Environmental Waters are Resistant to Beta Lactam Antibiotics

The gram-negative Aeromonas species from the family Aeromonadaceae occurs ubiquitously in aquatic environments. There are eight human and animal pathogenic species that can cause gastroenteritis after ingestion of contaminated food or water as well as wound infections after exposure to environmental waters. Of critical concern is when the Aeromonas spp. becomes resistant to extended-spectrum beta-lactam antibiotics. All beta-lactam antibiotics contain a four membered lactam, or cyclic amide, called a beta-lactam ring that can be hydrolyzed by the bacterial enzymes beta-lactamases. We hypothesize that the environment can serve as a reservoir for beta-lactamase genes. The aim of this study is to assess the presence and range of beta-lactam resistance in Aeromonas spp. from aquatic environments as well as to detect the occurrence of extended-spectrum beta-lactamases (ESBLs) in Aeromonas species. Water samples were collected from beaches and creeks in Orange County, California. Water samples were then filtered and plated on selective media. Matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry was used to obtain identification of Aeromonas species. In order to phenotypically detect ESBLs and resistance to beta-lactam antibiotics in Aeromonas bacteria, we conducted disk susceptibility tests in accordance to the Clinical and Laboratory Standards Institute (CLSI) guidelines. The production of ESBLs was phenotypically detected by comparing the inhibition zones of ceftazidime or cefotaxime plus clavulanic acid to ceftazidime or cefotaxime alone. 110 Aeromonas strains isolated were of the following species: 30% of A. hydrophila, 28% of A. caviae, 26% of A. culicicola, 6% of A. ichthiosmia, 5% of A. veronii, and 5% of A. allosaccharophila. 84 strains were beta lactam resistant as well as non-ESLB producing and showed high levels of resistance to the second-generation cephalosporin, Cefoxitin. 26 Aeromonas isolates were identified as ESBL-producing strains and showed high levels of resistance against the third generation cephalosporins, ceftazidime or cefotaxime. Of these 26 isolates, 46% were of the rare species A. culicicola, while 23% were of A. hydrophila and 19% were of A. caviae, which are both human pathogenic species. All of these ESBL isolates came from freshwater sites. Given the diversity of the Aeromonas spp. and the presence of beta-lactam resistance in nature, further investigations should be conducted in order to determine the specific genes that cause beta-lactam resistance in Aeromonas spp..