Prevalence of Antibiotic Resistance in Gram-Positive Bacteria from Raw Milk, and Antibiotic Resistance Gene Analysis in a Mastitis-Associated Streptococcus uberis Isolate from New Zealand

Abstract

Antimicrobial resistance is a growing global issue, with resistance to antibiotics increasing at a concerning rate. Resistance is an issue in both human and animal infection which has considerable implications for both human health and the economy. On dairy farms in New Zealand, antibiotic resistant bacteria can be found in the environment or as part of a cow’s natural microflora. If cattle become ill with mastitis, a mammary infection caused by a range of Gram-positive and Gram-negative bacteria, treatment with antibiotics can become complex if resistant bacteria are present. Therefore, antibiotic resistant bacteria surveillance in NZ dairy cattle is an important aspect of combatting this issue.

Antibiotic susceptibility testing on raw milk from NZ dairy farms using the Kirby-Bauer disc diffusion method was undertaken to measure the frequency of resistant bacteria. The extraction of bacterial DNA was performed with the intention of investigating the genome for genes that match its phenotypic resistance profile. Moreover, genomic analysis of the bovine mastitis isolate Streptococcus uberis was performed, using CARD and BLAST, to identify any relevant antibiotic resistance genes. Raw milk testing identified some putative Staphylococcus and Streptococcus/Enterococcus which demonstrated antibiotic resistance. Both the Staphylococcus and Streptococcus/Enterococcus isolates displayed 100% resistance to penicillin G. Staphylococcus general has a high level of resistance to β-lactam antibiotics whereas Streptococcus/Enterococcus were highly susceptible to the other β-lactams. Streptococcus/Enterococcus had moderate levels of resistance to novobiocin. The methicillinresistant Staphylococcus aureus (MRSA) did not diverge from the expected resistance profile. The quality control S. aureus had some atypical results, which may suggest phenotypic changes while in storage.

The CARD analysis for S. uberis found three antibiotic resistance genes, lnu(C) and patA/patB. BLAST analysis and a literature search showed lnu(C) is likely to be present in the S. uberis isolate but patA/patB is less likely to be present. In order to prove their presence and activity in the bacterium, further susceptibility testing and genome analysis is required.