Molecular characterization of antibiotic resistant bacteria and antimicrobial residues in the dairy milk in India

Introduction:
Food safety is a great concern in low- and middle-income countries. Milk and milk products are
staple foods that are consumed regularly by infants to adults in India and worldwide. However,
exposure to antibiotic-resistant bacteria in milk could pose a risk to public health. This project was
conducted to understand milk safety, by assessing antibiotic residues and resistant bacteria in the
milk from dairy farmers and vendors.
Methods:
The 729 milk samples were collected from two Indian states, and analyzed for the presence of food
pathogens following standard laboratory procedure. The disc diffusion antibiotic susceptibility testing
(ABST) was done to screen resistant bacteria. The resistant isolates by ABST were then studied by
Polymerase chain reaction (PCR) genotyping to detect methicillin resistance, extended-spectrum
beta-lactamases (ESBL), Metallo-beta-lactamases (MBL), and AmpC beta-lactamase’s (AmpC).
Antibiotic residues were assessed using rapid tests, and lateral flow tests.
Results:
The isolated gram-positive bacteria were found to be 74% (n=243) Staphylococcus spp. of which
only 6% (n=14) harbored methicillin resistance (14 mecA and 2 mecC genes). Among the isolated
gram-negative bacteria, 13% (n=43) of the isolates were resistant consisting of E. coli, Klebsiella
spp., and Shigella spp. 59 % (n=7) of them were carrying ESBL genes, 60% AmpC (n=26), 14%
MBL (n=6), 7% MBL+AmpC (n=3) and 2% ESBL+ AmpC (n=1). Antibiotic residues detected
included beta-lactam, chloramphenicol, quinolone, tetracycline, oxytetracycline, sulfa drugs,
streptomycin, and macrolides.
Conclusion:
The collected milk sample was found to be contaminated with 6% methicillin-resistant
Staphylococcus spp. and 13% with E. coli, Klebsiella spp., and Shigella spp with ESBL, MBL, and
AmpC resistance. On the other hand, the existence of antibiotic residues beyond MRL along with
pathogenic bacteria with a signature of resistance genes in farm animals as reservoirs and their
dissemination through the food chain poses a public health threat.