Aerococcus viridans - a true mastitis pathogen?

The article on Aerococcus viridans, an uncommon cause of bovine mastitis (intramammary infection), published in PLOS one, unfortunately had several drawbacks. Aerococcus viridans is a species which was in the conventional bacteriology often misidentified as esculin-positive streptococci. It is a saprophytic microorganism which is present in the barn environment, air and dust. Aerococci are found in bovine mastitic milk samples, but often in cultures with mixed growth and in samples suspected being contaminated (Owens et al., 1990; Devriese et al., 1999; cited in the article by Liu et al. 2015).

Satu Pyörälä
Professor
University of Helsinki, Faculty of veterinary medicine, Helsinki, Finland.

References

CLSI 2013. VETo1-S2. Performance standards for Antimicrobial disk and dilution susceptibility test for bacteria isolated from animals; Second Informational Supplement. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, PA, USA.

EUCAST 2015. Accessed in May, 2015.
http://mic.eucast.org/Euc...

Schwarz, S., Silleya, P., Simjee, S., Woodford, N., van Duijkeren, E., Johnson, A.P., Wim Gaastra, W. 2010. Editorial: Assessing the antimicrobial susceptibility of bacteria obtained from animals. Vet. Microbiol. 141, 1–4.
in samples suspected being contaminated (Owens et al., 1990; Devriese et al., 1999; cited in the article by Liu et al. 2015). Their role as true mastitis causing agents is not well documented. Bovine quarter milk sample is always prone to contamination, dispite the aseptic technique used in sampling. The larger than the standard volume of the milk cultured (50 µl) in the study by Liu et al. increases the risk for contamination. The authors ignore this point and consider all their isolates as true mastitis causing agents. No information about the isolates originating from pure growth samples or being isolated together with other species is given. Details on taking the quarter milk samples are missing from materials and methods. Definition for subclinical mastitis also deviates from the standard one: SCC over 500 000 per ml was used as threshold, but was it cow composite or quarter milk sample? For both of them this cut-off would need some basis why it was selected, because it is not the standard used. Futhermore, if milk production is decreased, mastitis is not subclinical. The two references used in this paragraph do not give any more information for the reader.

Another problem of this paper is related to in vitro susceptibility testing. Using clinical breakpoints aimed for other bacterial species is not appropriate. Furthermore, CLSI breakpoints are aimed for clinical purposes only (CLSI, 2013). They are derived from animal specific (or some organisms even human) microbiological, pharmacokinetic and pharmacodynamic data, which is not relevant for studies on in vitro susceptibility of epidemiological data sets as here (Schwarz et al. 2010). The authors used an outdated version of CLSI document. For this type of studies, epidemiological cut offs (ECOFF) must be used, such as those defined by EUCAST (EUCAST 2015). EUCAST ECOFFs are not veterinary specific but isolates originate from multiple sources and mainly from humans. CLSI breakpoints are generally higher than epidemiological ECOFFs. Comparing proportions of resistant isolates is completely meaningless if the cut offs are not the same. Molecular typing of the isolates in this study probably was adequate, but if the starting point of the study is weak, the message is useless.

Satu Pyörälä
Professor
University of Helsinki, Faculty of veterinary medicine, Helsinki, Finland.

References

CLSI 2013. VETo1-S2. Performance standards for Antimicrobial disk and dilution susceptibility test for bacteria isolated from animals; Second Informational Supplement. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, PA, USA.

EUCAST 2015. Accessed in May, 2015.
http://mic.eucast.org/Euc...

Schwarz, S., Silleya, P., Simjee, S., Woodford, N., van Duijkeren, E., Johnson, A.P., Wim Gaastra, W. 2010. Editorial: Assessing the antimicrobial susceptibility of bacteria obtained from animals. Vet. Microbiol. 141, 1–4.