Antibiotic susceptibility pattern and resistance genes in Salmonella strains isolated from cattle

Antibiotic resistance in Salmonella strains isolated from Pakistani cattle poses a significant concern for both animal and human health. This study, conducted at the Animal Health Department of the National Agriculture Research Center Islamabad (NARC) and the Zoology Department of Quaid-i-Azam University Islamabad, aimed to determine the antibiotic susceptibility pattern of Salmonella isolates recovered from Pakistani cattle and to identify associated antibiotic resistance genes.

A total of 100 cattle fecal samples suspected of containing Salmonella were collected from three commercial dairy farms selected based on owner willingness, prior history of enteric disease outbreaks, and accessibility. The bacterial isolates were processed for Salmonella confirmation using growth characteristics, biochemical tests, and InvA gene PCR. Antibiotic susceptibility was determined using the disk diffusion method according to CLSI guidelines, and isolates were screened by PCR for antibiotic resistance genes, including tetA, tetB, tetC, bla_TEM-1, bla_PSE-1, bla_CMY-2, bla_OXA,qnrA, qnrB, qnrC, qnrS, tetD, tetE, and tetG. Serotyping was not performed.

Out of 100 samples, 28 samples were confirmed as Salmonella. Phenotypic testing revealed high resistanceto oxytetracycline (100%) and doxycycline (100%), while ampicillin showed relatively lower resistance (64%). Other antibiotics displayed relatively lower resistance. All isolates were susceptible (100%) to ceftriaxone, cefixime, and sulfamethoxazole-trimethoprim, followed by gentamicin (71%), ciprofloxacin (58%), co-amoxiclav (57%), and imipenem (54%). Among the tested antibiotic resistance genes, tetA was highly prevalent which was demonstrated in 25% of the strains, followed by bla_TEM-1 (18%) and tetC (3.6%). No positive samples were obtained for bla_PSE-1, bla_CMY-2, bla_OXA, qnrA, qnrB, qnrC, qnrS, tetB, tetD, tetE, or tetG genes. Several isolates displayed phenotypic resistance without detection of the targeted genes.

This study provides insights into the antibiotic susceptibility pattern and prevalence of antibiotic resistance genes in Salmonella isolates from Pakistani cattle. The high resistance to oxytetracycline and doxycycline highlights the need for alternative treatment strategies. However, the findings should be interpreted with caution due to the limited sample size and the absence of serotyping, which restricted identification of specific Salmonella serovars. Continued surveillance and targeted control measures remain essential to mitigate the spread of resistant strains.