Discipline: mastitis; Keywords: Streptococcus uberis, SCC, clinical mastitis, biofilm, AMR, antimicrobials, susceptibility.

Antimicrobial resistance (AMR) is a global threat to human and veterinary medicine. Often it is not known to which antimicrobials specific mastitis-causing pathogens such as Streptococcus uberis are resistant or to which new antimicrobials they are sensitive to. A complicating factor is that one of the resistance strategies of many pathogens which makes it more difficult to treat than their planktonic counterparts, is biofilm formation. A further complicating issue is that virulence factors of Str. uberis have been discovered that can transfer resistance genes within members of the biofilm micro-community. To investigate further, the authors cited below determined the in vitro susceptibility of Str. uberis isolated from cow intra-udder infections on farms, to several antimicrobial agents used in the control of this disease. In addition, the study was also designed study the implications of biofilm formation.  

One hundred and eighty-five Str. uberis isolates were used in the study. The ability to produce biofilm were tested in all 185 isolates and antimicrobial susceptibility was determined in 174 of these isolates. An allocation of these isolates, based on SCC ranking was adopted: Group A were isolates from clinical mastitis cases; Group B from subclinical mastitis cases (SCC 300,000 cells/mL milk) and Group C from cases with intra-udder infections (SCC < 300 000 cells/mL milk). Biofilm production was categorised as: negative: OD ODc (all strains with OD values < 0. 0734); weak positive: ODc: < OD 2 × ODc (all strains with OD values > 0. 0734 and < 0. 1468); moderate positive: 2 × ODc < OD 4 × ODc (all strains with OD values > 0.1468 and < 0. 2936), and strong positive: OD > 4 × ODc (all strains with OD values > 0. 2936). Streptococcus uberis isolates were subjected to antimicrobial susceptibility of the more commonly used antimicrobial agents, namely: clindamycin (10ug), ampicillin (10ug), penicillin (10ug), oxacillin (10ug), cefoxitin (30ug), novobiocin, tetracycline (30ug), cephalothin (30ug), imipenem (10ug), vancomycin (30ug), ertapenem (10ug), chloramphenicol (30ug), ciprofloxacin (5ug), streptomycin (10ug), erythromycin (15ug) and trimethoprim (5ug).

The results showed that the least number of strong biofilm producers was in the group isolated from clinical mastitis cases and the most from the high SCC group. Most isolates from the group with clinical mastitis had moderate biofilm production. Those isolated from the high SCC group had mostly strong and those from the low SCC mostly low biofilm production. The percentage resistance in all antimicrobials tested were below 5% except for clindamycin (5.7%), daptomycin (5.7%), linezolid (8.0%), rifampin (8.0%) and tetracycline (6.9%). Of these, clindamycin and tetracycline is used for treatment of dairy cattle. Biofilm formation in the isolates tested for antimicrobial susceptibility were 47.3% weak positive, 34.9% moderate positive and 17.8% strong positive. Overall, little resistance was detected in the Str. uberis isolates. The highest susceptibility was detected in the group with moderate biofilm production (97.6%) and the least (92.2%) in the isolates that had strong biofilm production.

Conclusions: Biofilm formation by Str. uberis is of concern, but AMR not at this stage.


Sabelo, Petzer Inge-Marié and Karzis Joanne, 2022. Biofilm expression and antimicrobial resistance patterns of Streptococcus uberis species isolated from milk samples of South African dairy cows. Annual Report to Milk SA of project PRJ 0279-2021.