From feces to milk, bovine non-aureus staphylococci are everywhere

Text and picture: Ameline Wuytack

Non-aureus staphylococci (NAS) are worldwide the most common cause of intramammary infection (IMI) in lactating cows. This heterogeneous group of bacteria consists of 54 species of which at least 42 have been isolated from bovine-associated habitats such as quarter milk, teat apices and/or rectal feces from dairy cows. They are also abundantly present in the cow’s environment with every habitat and niche having its own specific NAS distribution. Research is still ongoing to unravel the species-specific ecology and epidemiology and to study the host-microorganism interaction. Some authors have demonstrated protective effects of certain NAS against major mastitis pathogens while others have reported detrimental effects on udder health such as the development of both (mild) clinical and subclinical mastitis.

Rectal feces as a possible new habitat was not yet investigated for bovine NAS.  Sterile samples from rectal feces were collected from clinically healthy cows in a commercial dairy herd and further examined to characterize fecal NAS isolates if present. Remarkably, half of the cows were shedding both so-called environmental and host-adapted NAS species. Acquired antimicrobial resistance was present in 90% of the fecal isolates and the most common antimicrobial resistance mechanism was β-lactamase production. Growth inhibition against mastitis-causing S. aureus, Streptococcus dysgalactiae, and/or S. uberis was found in 26% of the fecal isolates. The majority showed the ability to form biofilms and all fecal isolates were able to grow in conditions limiting the supply of oxygen and iron, mimicking the growth conditions in the mammary gland.

We also studied the species-specific NAS presence and distribution in bovine quarter milk, on teat apices and in rectal feces in a 2nd study enrolling 8 commercial herds. Non-aureus staphylococci were present in 33% and 98% of the quarter milk and teat apex samples, respectively. The presence of NAS in rectal feces, reported for the first time, was confirmed in 28% of the samples. Nine species, namely S. arlettae, S. auricularis, S. chromogenes, S. cohnii, S. devriesei, S. equorum, S. haemolyticus, S. hominis, and S. vitulinus were present in all 3 habitats in the same herd.

In addition, the virulence potential of NAS was evaluated by comparing their distribution amongst contrasting milk sample strata harboring NAS exclusively (quarters with a SCC ≤ 50,000 cells/mL and quarters with clinical signs) and by comparing the presence of 15 virulence genes in the NAS isolates. The occurrence of NAS was larger in milk samples originating from clinical mastitis (12%) than those originating from healthy quarters (4%). Therefore, we can conclude that some NAS have the potential to cause clinical mastitis, usually in a mild form. Studying the distribution of virulence genes in NAS isolates from the 2 contrasting milk sample strata supported this finding. Some NAS species tend to be more relevant for bovine udder health since samples exclusively harbouring S. chromogenes or S. haemolyticus, had a significantly higher quarter milk SCC than samples yielding no growthor Corynebacterium spp.

The concurrent presence of 9 NAS species in rectal feces, teat apices, and quarter milk in the same herds, indicate the potential of fecal NAS to cause NAS IMI, likely via teat apex colonization. Similar to other mastitis pathogens like Klebsiella spp., contamination of the cow’s environment can occur via shedding of the mastitis pathogen, thereby causing an opportunistic infection of the mammary gland. Still, both so-called environmental and host-associated NAS species were isolated from rectal feces, suggesting the possibility of a contagious transmission route as well. For S. chromogenes, S. cohnii, S. devriesei, and S. haemolyticus,the same strain type using random amplification of polymorphic DNA (RAPD) was found in rectal feces, teat apices, and quarter milk, suggesting that fecal NAS are a potential infection source for the mammary gland. For S. hominis and S. equorum,we were unable to confirm the presence of the same RAPD types in the three habitats.

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Dr. Ameline Wuytack graduated as a veterinarian in 2014 from Ghent University and followed a one-year educational internship program at the European College of Veterinary Public Health (ECVPH). In 2016, she started working on a PhD at the Mastitis and Milk Quality Research Unit (M-teamUGent) from the Faculty of Veterinary Medicine, Ghent University. During her PhD she also participated in international clinical trials in collaboration with pharmaceutical companies. Furthermore, she was engaged in the ambulatory clinic, the clinic of reproduction and obstetrics, and routine bacteriological culture of milk samples and uterus swabs. 







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