Antibiotic-Resistant Bacteria Discovered in Intestinal Microbiota of Ambystoma mexicanum
Bacterial isolates from the intestinal microbiota of the salamander species Ambystoma mexicanum (a common laboratory model organism and increasingly common household pet) were surveyed for resistance to a set of antibiotics. Fresh fecal deposits taken from the tanks of three separately-housed animals (two female, one male) were sampled on two occasions, eight days apart. Samples were plated onto general (Luria Bertani, LB) and selective/differential (Hektoen Enteric, HE, MacConkey- Lactose, MAC-Lactose, and Cetrimide, CT) agar and incubated at 30˚C for 24 hrs. A total of 93 isolates were characterized based on growth/phenotype on the HE, MAC-Lactose, and LB plates. Antibiotic sensitivities of these isolates were tested by patching them onto LB plates containing one of four different antibiotics at concentrations that inhibit the growth of laboratory Escherichia coli strains. Of the 93 isolates, 88% were resistant to ampicillin, 66% were resistant to chloramphenicol, 27% were resistant to kanamycin, and 1% was resistant to tetracycline. Of these isolates, 26 isolates were resistant to antibiotics through acquired resistance. The identity to the genus level was tentatively determined for 83% of the isolates based on 16S rRNA gene sequences, and a phylogenetic tree was created which showed clustering of related isolates. The variety of phenotypes and responses to antibiotics in this pilot study suggest the intestinal microbiota of Ambystoma mexicanum is sufficiently diverse to justify a broader, metagenomics survey to further evaluate species composition and variation of genes encoding resistance to beta lactam antibiotics in this habitat.