Characterization of Pseudomonas aeruginosa affecting on somatic cell counts and antibiotics efficiency of bovine mastitis

Abstract

Pseudomonas aeruginosa is one of the causative pathogens of bovine mastitis. Most P. aeruginosa strains possess the type III secretion system (TTSS), which may increase somatic cell counts (SCCs) in milk from mastitis-affected cows by causing inflammatory responses and cell death. Moreover, most of P. aeruginosa cells can form biofilm, thereby reducing antibiotic efficacy through multiple mechanisms. In this study, the presence and effect on SCCs increase of TTSS-related genotypes among 122 P. aeruginosa isolates obtained from raw milk samples from mastitis-affected cows, their biofilm-forming ability, antibiotic susceptibility at planktonic and biofilm status, and genetic relatedness were investigated. In spite of heterogeneity confirmed by multiple-locus variable number tandem repeat analysis, most of isolates (99.2%) possessed at least 1 TTSS-related genes

exoS, T, Y, and U. Among the TTSS-positive isolates, 69.4%, 8.3%, and 4.9% of isolates were invasive (exoU-/exoS+), cytotoxic (exoU+/exoS-), and cytotoxic/invasive type (exoU+/exoS+), respectively, based on the carried TTSS-related genes. Raw milk containing isolates possessed exoU showed higher SCCs than raw milk containing exoU-negative isolates. According to results of microtiter plate biofilm formation assay, most of isolates (97.5%) were biofilm producers. Majority of isolates showed gentamicin, amikacin, meropenem, and ciprofloxacin susceptibility at planktonic status. However, the susceptibility was decreased at biofilm status. Based on minimum biofilm eradication concentration (MBEC)/minimum inhibitory concentration (MIC) ratios, range of change in antibiotic susceptibility varied widely depending on the antibiotics (from ≥3.1-fold to ≥475.0-fold). These results indicate that the majority of P. aeruginosa isolates from mastitic raw milk in Korea had the genotype that may be related to SCCs increase. The efficiency of antibiotics therapy of bovine mastitis involving P. aeruginosa could be improved if MBEC as well as MIC test results would be considered.