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* Department of Veterinary Pathology, Hygiene and Public Health, University of Milan, via Celoria 10, 20133 Milan, Italy
Institute of Agricultural Biology and Biotechnology (IBBA), Consiglio Nazionale delle Ricerche (CNR), Milan, Italy
Department of Veterinary Sciences and Technologies for Food Safety, University of Milan, via Celoria 10, 20133 Milan, Italy
1 Corresponding author: paolo.moroni{at}unimi.it
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ABSTRACT |
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 TOP ABSTRACT ACKNOWLEDGEMENTSREFERENCES |
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Key Words: Staphylococcus aureus • bovine • minimum inhibitory concentration
Bovine mastitis is the most costly disease to the dairy industry worldwide, with losses estimated at 2 billion dollars per year in the United States alone. These relevant economic losses are attributable to rejected milk, reduced milk quality, early culling, drug costs, veterinary expenses, and increased labor costs (Hoblet et al., 1991; Gruet et al., 2001). The disease is the most frequent reason for the use of antimicrobial agents on dairy farms (Erskine, 2000). Intramammary infection sustained by Staphylococcus aureus may result in clinical or subclinical mastitis and is usually associated with increased SCC. Appropriate treatment of mastitis during the lactation or dry period is an important component of any mastitis control program, but the outcome for treatment of mastitis caused by Staph. aureus is variable and the probability of curing the disease is not high, primarily because of poor distribution of the drug in the inflamed udder and the occurrence of staphylococci resistant to antimicrobial agents (Pyörälä and Pyörälä, 1994). During lactation, the cure rate of subclinical mastitis ranges widely, and this variability can be due to the choice of antimicrobial agent as well as to factors associated with the infected cow and the quarter. Therefore, cure is likely not a random event (Schukken et al., 1994). Antimicrobial therapy is a primary tool for controlling staphylococcal mastitis, and antimicrobial susceptibility tests can guide the veterinarian in selecting the most appropriate antimicrobial agent for treatment of IMI by Staph. aureus. However, despite a variety of available antimicrobial agents, success in the treatment of Staph. aureus mastitis, particularly during lactation, is still very low. In fact, Staph. aureus pathogens have many characteristics that make them difficult targets for antimicrobial therapy (Sol et al., 2000). For example, they can penetrate the phagocytic cells and survive inside them. This process shields the pathogens from some of the activity of antimicrobial agents, even with drugs that can penetrate the cells. The objective of the present study was to evaluate the antimicrobial resistance patterns of Staph. aureus isolates collected from IMI in cows from 44 commercial herds in Italy.
Sixty-eight isolates of Staph. aureus taken from individual quarters of 68 cows with subclinical mastitis were used. The isolates were obtained during 2004, from milk samples collected from animals belonging to 42 dairy herds from different regions of Italy. Animals were selected on the basis of having quarter milk SCC greater than 400,000 cells/mL but no sign of clinical mastitis. Herds were selected to represent different prevalences of Staph. aureus IMI, ranging from 1 to 100%. The number of isolates for each herd was determined according to the number of lactating animals in each herd: One single isolate was collected from 26 small herds (
50 lactating cows), 2 isolates were collected from 11 medium-sized herds (100 lactating cows), and more than 2 isolates were collected from 5 large herds (>100 lactating cows). All isolates were identified on the basis of morphology, hemolysis pattern, and gram staining. The gram-positive cocci were tested for catalase and coagulase production. The species were identified by biochemical tests and by the API Staph System (BioMérieux, Rome, Italy) and were then stored at –70°C in a nutrient broth enriched with 15% glycerol.
In the present study, the antibiotics were selected by considering the approved and most frequently used drugs for the treatment of bovine IMI in Italy, even if these drugs were not representative of a particular antibiotic class. The antimicrobial agents selected and provided by manufacturers as powders were penicillin G (PEN), ampicillin (AMP), amoxicillin (AMX), AMX + clavulanate, cloxacillin (CLX), cephalonium (CFL) and cefoperazone (CFP), kanamycin, oxytetracycline, doxycycline, and lincomycin (LIN). The antimicrobial agents were dissolved in suitable solvents to make stock solutions and then diluted in sterile distilled water according to the methods recommended by the Clinical Laboratory Standards Institute (2002). Minimum inhibitory concentration tests were performed according to the microdilution broth method, as recommended by the Clinical Laboratory Standards Institute (2002a), using U-bottomed 96-well microtiter plates. Serial 2-fold dilutions of the antimicrobial agents were prepared starting from the stock solution of each drug. The dilution schemes differed according to the antimicrobial agent. Inocula were prepared by diluting an overnight (16 to 18 h) Mueller–Hinton broth culture in buffered saline solution to a density of 0.5 on the McFarland turbidity scale and finally diluting it again 40-fold before testing. The MIC was defined as the lowest concentration of the antimicrobial agent at which the bacterial growth was completely inhibited. A reference strain (Staph. aureus, ATCC 29213) was inoculated as a control in each plate. The MIC data were summarized, calculating the MIC values for which the isolates were equal to or below 50 and 90% (MIC50 and MIC90, respectively), as well as the minimum and maximum MIC values (range). Resistance and susceptibility, for most of the antimicrobial agents tested, were determined according to Clinical Laboratory Standards Institute (2002a) MIC breakpoints for veterinary pathogens. The interpretive criteria, however, were based on MIC data and drug pharmacokinetic data obtained in humans (taken from Clinical Laboratory Standards Institute, 2002b). Staphylococcus aureus was also tested for ß-lactamase production by the nitrocefin test (Cefinase, Becton, Dickinson and Co., Sparks, MD).
Numerous data are available in the literature on the susceptibility to antimicrobial agents of Staph. aureus isolates collected from IMI of dairy cattle (Craven et al., 1986; Watts and Yancey, 1994; Owens et al., 1997; Makovec and Ruegg, 2003; Pengov and Ceru, 2003; Tikofsky et al., 2003). Nevertheless, obtaining continually updated MIC values is important to prevent the use of ineffective antimicrobial drugs. Table 1
reports MIC50 and MIC90 values of the selected antibiotics against the Staph. aureus isolates examined in the present study. All values obtained with the control strain were within the expected ranges for all antimicrobial agents tested. Of the 68 isolates evaluated, none were susceptible to all antibiotics and 64 (94%) were resistant to 3 or more antimicrobial agents. No isolates were resistant to CLX. The ß-lactams (penicillins and cephalosporins) are widely used for intramammary treatment of bovine mastitis, but in the present experiment we observed a very poor activity of PEN (MIC50 = 0.5 and MIC90 = 2,000 µg/mL). The MIC90 for PEN reported in previous studies ranged from <0.06 to >100 µg/mL (De Oliveira et al., 2000; Erskine et al., 2004). In our study, 47 isolates (69%) of Staph. aureus were PEN resistant. This proportion was greater than those reported for comparable studies in Argentina (40%; Gentilini et al., 2000), the United States (38.4 to 60.9%; Erskine et al., 2002), and Finland (50%; Myllys et al., 1998), but was lower than that reported for strains isolated from mammary parenchymas of slaughtered dairy cows in Brazil (75%; Costa et al., 2000). Similarly, in the penicillin group, AMP and AMX had very poor in vitro activity (MIC50 = 2 and 4 µg/mL, and MIC90 = 500 and 1,000 µg/mL, respectively), and 100 and 98.5% of the isolates were resistant to these respective antimicrobial agents. Results from other studies for MIC90 for AMP differed remarkably from our findings; they ranged from only 0.5 to 4 µg/mL (Watts and Salmon, 1997; De Oliveira et al., 2000). This high level of resistance was probably related to the presence of strong ß-lactamase producers among the tested staphylococcal isolates. The in vitro data confirmed the influence of ß-lactamase production on the microbial susceptibility to ß-lactams in general and to PEN in particular. In fact, the difference between MIC50 and MIC90 values, with reference to PEN, AMP, and AMX, correlates very well with the identification of 28 ß-lactamase–producing isolates (58%). Recalculation of the MIC90 without these strains yielded values of 0.5 µg/mL for PEN and 4 µg/mL for both AMP and AMX. On the other hand, the ß-lactamase–resistant CLX and amoxicillin + clavulanate (a widely used ß-lactamase inhibitor) were both highly effective, with MIC50 of 0.25 and 1 to 0.5 µg/mL and MIC90 of 0.5 and 8 to 4 µg/mL, respectively.
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The lincosamide antimicrobial agents (e.g., LIN and clindamycin) act by inhibiting RNA-dependent bacterial protein synthesis (Yao and Moellering, 1995). Lincomycin showed an MIC90 of 250 µg/mL, and this value was greater than those previously found in other countries. For example, LIN MIC90 ranged from 16.0 to 64 µg/mL for isolates from the United States, Ireland, Iceland, and Germany and from 1.0 to 8.0 µg/mL for isolates from Denmark, England, Norway, Sweden, and Finland (De Oliveira et al., 2000). The LIN MIC90 value obtained for the strains tested in this study may be linked with their carriage of the erm gene, which encodes resistance to lincosamides, macrolides, and strep-togramine B antimicrobial agents (Leclercq and Courvalin, 1991). Tetracyclines and aminoglycosides are broad-spectrum antibiotics widely used to treat respiratory and other diseases in cattle. Because of this widespread use, tetracycline and aminoglycoside resistance, coded by a wide variety of determinants, was demonstrated by the high MIC90 observed in the present study.
Several factors other than antimicrobial usage can influence the overall susceptibility patterns of mastitis pathogens. Scar tissue in the udders of cattle chronically infected by Staph. aureus often prevents the penetration of antimicrobial agents (De Oliveira et al., 2000). Therefore, the general recommendation is to cull all animals with chronic Staph. aureus IMI. The control of IMI sustained by Staph. aureus should involve the best management practices and selective antimicrobial usage. Unfortunately, most antimicrobial agents used in veterinary medicine still rely on interpretive criteria developed for humans, and the validity of these interpretive criteria for categorizing veterinary pathogens as susceptible or resistant has not been established (Watts and Yancey, 1994). Currently, only pirlimycin and a penicillin–novobiocin combination have had interpretive criteria developed using MIC data generated with mastitis pathogens. Interpretation of antimicrobial susceptibility data for the remaining compounds relies on interpretive criteria developed with human data. The interpretive criteria used for categorizing isolates as susceptible or resistant are based on human data for most of the drugs tested in this study. Thus, the usefulness of susceptibility data is limited to monitoring the percentage of Staph. aureus with MIC above a threshold value, and these values may not be used to predict clinical efficacy. The percentage of resistance data presented in this study was used for comparative purposes but not as an indicator of the actual resistance level. The experimental tests performed showed important in vitro activity against the Staph. aureus isolates of the majority of antimicrobial agents currently used in Italy for control of IMI. However, we consider it necessary to develop new interpretive criteria for studying specific mastitis pathogens and for predicting clinical efficacy in all those situations in which, as in cases of mastitis caused by Staph. aureus, scar tissue barriers or other pathological or physiological factors can reduce the in vivo efficacy of the drugs.
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ACKNOWLEDGEMENTS |
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FOOTNOTES |
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Received for publication October 17, 2005. Accepted for publication February 28, 2006.
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