Mastitis is a major infectious disease of dairy cattle throughout the world, which consisted of clinical mastitis and subclinical mastitis. It leads to losses associated with the reduction of milk production and quality, treatment cost, milk discard and animal mortality
(Bianchi et al., 2019; Saravanan et al., 2015). Clinical mastitis shows several clinical signs including abnormal udder size and milk secretion, reduced milk production, whereas subclinical mastitis may not show any signs on the udder or changes in the milk
(Ashraf et al., 2018; Sharma et al., 2012). Enormous economic losses caused by mastitis was observed worldwide
(Bianchi et al., 2019; Kappeli et al., 2019). In US dairy, the total cost in the average case of clinical mastitis was $444
(Rollin et al., 2015). An annual loss of EUR 9.03 billion associated with summer mastitis was reported in the UK dairy industry
(Halasa et al., 2007).
Mastitis pathogens can be divided into two groups including contagious and environmental pathogens.
Streptococcus uberis has been identified as the most frequently isolated environmental or contagious pathogen from clinical and subclinical mastitis
(Cameron et al., 2016; Wente et al., 2019). It can colonize animals and their environment without the need for a specific living environment such as udder milk. Previous research reported that 51.6% and 63-85% of
S. uberis were isolated from the skin of dairy cows and environmental samples including water, soil and bedding materials, respectively (
Kromker, 2014).
S. uberis can produce biofilms that might result in persistent intramammary infections and treatment failure through increased resistance to antibiotics (
Schönborn et al., 2017). Thus,
S. uberis, the difficult-to-handle pathogen, was considered as the barrier to the control of bovine mastitis as the epidemiology was not completely understood
(Tomazi et al., 2019; Wente et al., 2019).
Antimicrobial usage is the most effective method for treating mastitis caused by bacteria. Antimicrobial treatment was reported in most cases of mastitis worldwide
(Song et al., 2020; Wente et al., 2019). However, the treatment of environmental and contagious pathogens of
S. uberis presents a challenge for the management of dairy cattle
(Tian et al., 2019). The overuse and misuse of antimicrobials play a significant role in increasing resistance to them. The emergence of resistant isolates occurred in the gene mutation or horizontal transmission of antimicrobial resistance genes (ARG) from another microorganism
(McDougall et al., 2020). In recent decades, resistance to antimicrobial agents has become a global problem for both human and animal health. Antimicrobial resistance (AMR) in dairy cattle can impact humans due to the potential dissemination of AMR pathogens to humans
via consumption of infected dairy products or contact with infected dairy cattle
(Molineri et al., 2021). Additionally, animal waste with resistant isolates and antimicrobials could impact the environment. Therefore, monitoring the antimicrobial susceptibility of
S. uberis is essential to guide the veterinaries in selecting the most appropriate antimicrobials for treating the disease. The most frequently used antimicrobial classes for treating bovine mastitis caused by
S. uberis are β-lactam, aminoglycosides, tetracyclines and macrolides (
El Garch et al., 2020;
Loch et al., 2005; McDougall et al., 2020). This review aimed to describe the phenotypic resistance to antimicrobials in
S. uberis collected in worldwide between 2000-2020.
Prevalence of pathogens
A variety of pathogens can cause mastitis in dairy cows, the most common contagious mastitis pathogens were
Staphylococcus aureus and
Streptococcus agalactiae (
Juozaitienë et al., 2020;
Shome et al., 2012). In South America,
S. agalactiae showed 35% frequencies
(Kabelitz et al., 2021). S. aureus was isolated from 51.2% mastitis milk samples from Ethiopia
(Abebe et al., 2016), whereas 10.17-16.67% of
S. aureus was detected in Finland (
Pitkälä et al., 2004). However, several studies have shown that the most prevalent pathogens causing clinical mastitis in cows are usually organisms that originate from the environment. Common environmental organisms include coagulase-negative staphylococci (CNS),
S. uberis,
Streptococcus dysgalactiae,
Klebsiella spp. and
Escherichia coli (Sharma et al., 2012). The research reported in Wisconsin showed that the most prevalent pathogens were
E. coli (22.5%), followed by environmental streptococci (12.8%),
Klebsiella spp. (6.9%) and CNS (6.1%)
(Oliveira et al., 2013). Song et al., (2020) reported that the most frequently isolated pathogens from mastitis milk samples were environmental bacteria (67.53%).
S. agalactiae and
S. uberis were more prevalent in farms using sand bedding. Environmental streptococci, especially
S. uberis ranks among the prime causative agents of mastitis around the world. Mastitis caused by
S. uberis showed an increasing trend in dairy farms (
Kromker 2014). Thus, this review aimed to summarize the antimicrobial susceptibility of
S. uberis.
Antimicrobial resistance
Resistance to β-lactams
S. uberis has historically been regarded as highly susceptible to β-lactam antimicrobials because it didn’t produce β-lactamase. Thus, these antimicrobials were the first choice for this disease infected by the bacteria. In general, published data revealed that
S. uberis was highly susceptible to β-lactam antimicrobials including ampicillin, penicillin G, amoxicillin, ceftiofur, oxacillin, cloxacillin (Table 1). However, the ongoing pan-European antimicrobial susceptibility monitoring programme (VetPath) in Europe reported increased median minimum inhibitory concentrations (MIC
50) value for penicillin G in
S. uberis from 0.03 to 0.12 during 2002 to 2012 (
de Jong et al., 2018;
Thomas et al., 2015). Increased MIC of oxacillin in
S. uberis in New Zealand was observed as well
(McDougall et al., 2014). In addition, phenotypical resistance to penicillin was isolated from Estonia (0.4%), New Zealand (1%), Slovakia (10.5%), Canada (5.8%), Switzerland (7.7%) and Korea (8.1%). High levels of resistance to oxacillin were observed in Switzerland (64.7%) and Korea (33.3%). AMR of
S. uberis strains vary among studies due to various elements including the sample selection and size, the population of isolates, the time and place of the study, the methods of susceptibility test, the breakpoints and references used and the analyses carried out. Meeting all these criteria makes studies comparison difficult. However, this review can provide useful references to address the problem. Penicillin-binding proteins (PBPs) are important proteins involved in the construction of cell wall peptidoglycan. Resistance against β-lactam antimicrobials, especially for gram-positive cocci, was mainly caused by altered PBPs.
McDougall et al., (2020) investigated the genetic basis of increasing MIC for β-lactam of
S. uberis. The results indicated that the presence of
pbp substitutions was associated with decreased susceptibility to β-lactam antimicrobials and lower cure rate outcomes following antimicrobial therapy for clinical mastitis. Similarly, the presence of β-lactamase enzymes was frequent with high MIC values of β-lactam antimicrobials for
S. uberis isolates
(Velez et al., 2017). However,
Kaczorek et al., (2017) reported no relationship between
blaZ and increased MIC values in Poland. Therefore, more research is needed to determine the mechanism of β-lactam resistance of
S. uberis.
Resistance to macrolides
Macrolides are commonly used for the treatment of bovine mastitis caused by streptococci. Previously, the frequency of resistance to macrolides for streptococci was less than 10% in dairy cattle
(Loch et al., 2005). S. uberis was highly susceptible to erythromycin. Although erythromycin resistance frequency varied in different studies, most frequencies were 1.9%-24% and the highest resistance frequency (34.3%) was observed by
Nam et al., (2009) in Korea.
Resistance to macrolides can be caused by several mechanisms such as the presence of ribosomal methylase genes [
erm (A),
erm (B),
erm (C)] and efflux pump genes [
mef (A),
mef (E),
msr (A)]
(Rossolini et al., 2017). The
erm genes are currently the most common functional gene responsible for the resistance to macrolides observed in streptococci
(Entorf et al., 2016). The
erm(B) was most frequently detected among erythromycin-resistant streptococci
(Entorf et al., 2016), which in accordance with the finding in France
(Haenni et al., 2011). According to
Loch et al., (2005) all
S. uberis isolates with MIC≥16 for erythromycin were positive for
erm(B) gene. Similarly,
Kaczorek (2017) reported that the
erm(B) gene predominates in
Streptococcus spp. Previous research demonstrated that it might be due to the horizontal gene transfer among the bacteria of genus
Streptococcus from cattle or farm environments
(Loch et al., 2005).
Resistance to tetracyclines
Tetracycline is a broad-spectrum antimicrobial against a wide variety of gram-positive and gram-negative bacteria. It is extensively used in animals, which is also important for treating the same bacteria in humans. Thus, it is not surprising the less effective antimicrobial therapy caused by the resistant pathogens in dairy cattle. Resistance to tetracycline varied in different studies. Several studies have reported that high levels of resistance to tetracycline were found in most countries including Estonia (19.7%), Canada (38.6%), Danish (21.3%), France (18.1%), Switzerland (28.4%), Czech (63.2%), Germany (42.3%), Portugal (60%), Poland (35.3%) and Korea (57.6%), whereas resistance to tetracycline was 0.9% in New Zealand and 8.8% in Slovakia. Additionally, resistance trend to tetracycline was linear increase from 15.7% in January 2006 to 20.4% in December 2016 in France
(Boireau et al., 2018). A similar trend was found in Europe (
de Jong et al., 2018;
El Garch et al., 2020;
Thomas et al., 2015). Resistance to tetracyclines in
Streptococcus spp. can be due to several different mechanism such as ribosomal protection [
tet (M),
tet (O)], tetracycline efflux system [
tet (L),
tet (K),
tet (40)]
(Rossolini et al., 2017). However, limited research based on the resistance mechanism in
S. uberis was reported.
Resistance to aminoglycosides
Aminoglycosides may be used to the treatment of bovine mastitis. The present data showed that
S. uberis was highly resistant to aminoglycosides antimicrobials. Resistance to gentamicin showed a broad range (0-99.7%), whereas higher level of resistance to streptomycin was found in all published research. The research observed by
Kaczorek et al., (2017) indicated that
Streptococcus spp. showed naturally low effective against aminoglycosides antimicrobials.
Resistance to other antimicrobials
Several studies reported the resistance to sulfamethoxazole/trimethoprim in Estonia (3.2%), France (9.3%), Slovakia (5.3%), Finland (1.6%) and New Zealand (12.7%). Pirlimycin was used to treat bovine mastitis caused by gram-positive cocci, which is approved for veterinary use (
El Garch et al., 2020). Resistance to pirlimycin was detected in Canada (21.4%), Germany (18.9%), Europe (15.9%) and Portugal (53.3%). The study observed by
Pol et al., (2007) indicated that increased MIC of pirlimycin was associated with increasing exposure to defined daily doses of antimicrobial.
Antimicrobial resistance over time
This review reported the phenotypic resistance to antimicrobials in
S. uberis collected from worldwide between 2000-2020. In this period, the resistance trend to tetracycline was linear increase from 15.7% in January 2006 to 20.4% in December 2016 in France
(Boireau et al., 2018). Similarly, the prevalence of AMR to tetracycline in Europe presented an increase trend from 28.7% in 2002 to 37.5% in 2016 (Fig 1). In Poland, the results showed a similar prevalence of AMR to tetracycline from 2010 to 2015. However, it is difficult to compare as different methods were used in the research. For erythromycin, the AMR prevalence in Europe was increased from 18.8% to 23.9% during 2002 to 2016.
The development of AMR is considered a problem to public health. According to the findings in this review, the increased AMR resistance for several antibiotics was detected as antibiotic usage in dairy farms. However, most research could not reflect the trend because test method, sampling and sample size, statistical analysis may influence the results.
Monitoring of antimicrobial usage and AMR in bacteria isolated from food-producing animals are essential for determining the emergence of resistant bacteria and providing strategies to reduce the spread of resistance. This article reviewed the AMR of
S. uberis associated with bovine mastitis worldwide. It is difficult to compare the AMR patterns among countries as several factors may affect the results including differences in the methods performed in various research (disc diffusion or MIC), lack of interpretive criteria, differences in sample collection (
El Garch et al., 2020;
Molineri et al., 2021). Systematic review and meta-analysis reported by
Molineri et al., (2021) demonstrated that studies using disc diffusion method presented higher AMR of
Staphylococcus aureus than those using MIC. The AMR prevalence of cefoxitin, erythromycin, gentamycin and oxacillin was 10.57, 5.47, 3.94 and 2.94 times higher for disc diffusion method than for MIC, respectively. Currently, only a few clinical breakpoints were used for antimicrobials against
S. uberis. Methodology should be improved for surveillance data and optimize mastitis treatment. Additionally, most research on AMR of
S. uberis was reported in Europe according to the present review (Fig 2). More surveillance is needed in other regions.
Antimicrobials are commonly used to treat bovine mastitis. The development of AMR is associated with antimicrobials usage in dairy farms. However, few studies investigated its relationship and only a few studies have compared the trends in AMR of
S. uberis from the same geographical area and method
(Boireau et al., 2018; de Jong et al., 2018;
El Garch et al., 2020;
Thomas et al., 2015). It is necessary to continuously monitor the AMR of bovine mastitis pathogens worldwide and guide the veterinarians or farms to improve the management practices regarding antimicrobials usage.
Prevention of mastitis in dairy cows is better than treatment. The vaccine is considered an effective measure to prevent bacterial mastitis. However, a vaccine that prevents
S. uberis mastitis is not available. Therefore, the control and prevention measures need to target the relevant infection risks, for example, poor environmental hygiene or non-use of teat-sealants in the dry period, bedding materials, dirty udders
(Klaas et al., 2018). The incidence of environmental streptococcal mastitis in the dry period is 5.5 times higher than in other breast feeding periods. Farmers commonly use antimicrobials to treat it in the dry period. The basis of all prevention measures for streptococcal mastitis is to limit the exposure of the nipple ends. Therefore, the lying and bedding area should be kept clean, especially dry and avoid placing too deep straw bags. Sand or sawdust is preferred
(Hillerton et al., 2003). It is recommended that cows stand for at least one hour after milking until the teat canal is properly closed. In addition, the infected cattle should be isolated to avoid the spread of bacteria in the herd. Good milking operations such as teat disinfection and drying, regular cleaning and inspection of the milking machine, could reduce the bacterial contamination of individual cows. The application of these measures has reduced the incidence of streptococcal mastitis by 50%
(Hogan et al., 2003).
