Multilocus Sequence Genotype Heterogeneity in Streptococcus uberis Isolated from Bovine Mastitis in the Czech Republic

SIMPLE SUMMARY: Bovine mastitis is a serious problem for dairy farmers, resulting in great economic losses. A large number of antimicrobials are used to treat mastitis, contributing to the spread of resistance. Streptococcus uberis is an important environmental pathogen responsible for a significant proportion of subclinical (asymptomatic) and clinical intramammary infections in many countries. This pathogen is present in the environment of cows, colonising multiple body sites of the cow, including the mammary gland. Isolates may produce virulence factors that enable the bacteria to infect the mammary gland, resist the defence mechanisms of the mammary gland, and persist inside the gland. S. uberis isolates differ in virulence and the level of antimicrobial resistance, posing a challenge to controlling S. uberis infection. Therefore, it is necessary to study the biology and genetics of this pathogen to be able to help farmers and veterinarians to implement effective targeted measures against S. uberis mastitis. ABSTRACT: The ubiquitous occurrence and high heterogeneity of Streptococcus uberis strains cause difficulties in the development and implementation of effective control strategies in dairy herds. In this study, S. uberis strains from 74 farms, obtained predominantly from subclinical, acute, and chronic recurrent mastitis, as well as from udder surface swabs and milk from healthy udders, were analysed for their genetic diversity using multilocus sequence typing (MLST). Isolates were tested for the presence of the genes encoding the virulence factors using polymerase chain reaction. Antibiotic susceptibility testing was performed using a microdilution assay including 14 antimicrobials. The virulence profiles and antimicrobial (AMR) profiles of the isolates were assembled and the overall heterogeneity was evaluated. Among the 124 isolates, 89 MLST genotypes, 7 different virulence profiles, and 12 AMR profiles were identified. The large number of different MLST allelic profiles in this study points to the high heterogeneity of strains in dairy herds in the Czech Republic. Isolates of a certain MLST genotype may possess a different set of virulence factor genes. We detected up to three different resistance profiles within a single MLST genotype. The results of our study showed that fully susceptible isolates coexisted with resistant or even multiresistant isolates in the same herd. Multiple genotypes within a herd were detected on many farms (up to seven MLST genotypes and four AMR profiles in one herd). This heterogenic population structure might suggest that environmental transmission is the predominant route of infection in herds in the Czech Republic.