Beta-Lactamase Genes in Escherichia Coli from Aquatic Environments

Beta-lactams make up about 60% of all antibiotics used in medicine. They function by inhibiting cell wall synthesis in bacteria. E. coli are gram-negative bacteria naturally found in the gastro-intestinal tract of mammals. Antibiotic resistance has become a major health concern as current antibiotics are becoming less effective against bacterial infections, such as urinary tract infections and gastroenteritis caused by pathogenic E. coli. The major mechanism of antibiotic resistance that E. coliemploys is via beta-lactamases, enzymes that hydrolyze the beta-lactam ring of beta-lactam antibiotics. Genes encoding these enzymes are often carried on plasmids, ensuring their rapid spread through horizontal gene transfer. Although antibiotic resistant genes in healthcare settings are well studied, little is known about these genes in bacteria from natural environments. The purpose of this study is to describe the beta-lactamase genes in strains of E. colifrom the environment and to determine their frequencies. In this study, water samples were collected from different bodies of water in Southern California. Eighty-four bacterial strains were isolated using filtration and selective media and identified using MALDI-TOF mass spectrometry. DNA was isolated and used to screen for beta-lactamase genes through PCR and agarose gel electrophoresis. Seventeen beta-lactamase genes were probed: CTX-M, SHV, CARB, TEM, GES, PER, VEB, KPC, IMP, VIM, CMY, ACC, FOX, MOX, DHA, EBC, and OXA.  The detected genes were sequenced and analyzed using Mega5 software and NCBI tools. Four of our CTX-M type alleles were blasted through GenBank to observe their geographical distributions. The most common gene found was TEM (51%) followed by CTX-M (29%), CMY (25%), OXA (14%), CARB (1%), and ACC (1%). Other tested genes were not detected. TEM, CTX-M, CMY, ACC, and OXA are extended-spectrum beta-lactamases, which confer resistance to the majority of beta-lactam antibiotics, such as penicillins, cephalosporins, and monobactams. Identical variants of CTX-M were found around the world. The high frequency of beta-lactamase genes in Southern California indicates that aquatic environments may be a hotbed for antibiotic resistance. Finding these genes is concerning because of a possibility for gene exchange among diverse bacteria that come in contact with humans. Finding these genes in different bacteria around the world is a strong indication of horizontal gene transfer. Our study calls for the necessity of regular monitoring of environmental samples for the presence of antibiotic resistance genes to determine their source and dynamics.