Sensitivities of foodborne pathogens to pressure changes

Eight foodborne pathogens were suspended in ultrahigh-temperature whole milk and treated at pressure levels of 0.1 to 690 MPa at 21.5 degrees C for 10 min. There was no clear trend in pressure resistance between gram-negative and gram-positive organisms. The order of the single strains tested, from most to least pressure sensitive, was Vibrio parahaemolyticus < Yersinia enterocolitica < Listeria monocytogenes < Salmonella enterica serovar Typhimurium < S. enterica serovar Enteritidis < Escherichia coli O157:H7 approximately equal to Staphylococcus aureus < Shigella flexneri. For each organism there existed a pressure range in which log(number of survivors) had a near linear relationship when plotted versus treatment pressure level. In this study, a decimal reduction pressure (Dp) value was defined and used to measure the sensitivity of these pathogens to pressure changes. L. monocytogenes and V. parahaemolyticus were most sensitive to pressure changes, and S. flexneri was most resistant. The D(P) values were 16.3 MPa for L. monocytogenes, 21.7 MPa for V. parahaemolyticus, and 127.0 MPa for S. flexneri. The most pressure-resistant gram-negative bacterium, S. flexneri, and most pressure-resistant gram-positive bacterium, S. aureus, were treated at 50 degrees C and pressures of 0.1 to 650 MPa for 10 min. High temperature considerably enhanced pressure inactivation of these two organisms and affected their sensitivities to pressure changes. The effect of treatment time on the D(P) values of L. monocytogenes and V. parahaemolyticus was also determined, and it was found that it did not significantly affect their D(P) values.     

Relationship between occurrence of mastitis pathogens in dairy cattle herds and raw-milk indicators of hygienic-sanitary quality

Mastitis is an inflammation of the mammary glands and in most cases it is caused by the presence of microorganisms. High mastitis rates in dairy cattle herds can cause an increase in total microorganism counts of bulk tank milk. The present paper was aimed at verifying whether the occurrence of mastitis in dairy cattle herds is reflected in raw-milk indicators of hygienic-sanitary quality. To observe the correlation among the analysed variables, we performed a logarithmical transformation (log10) of different indicator counts of raw milk and compared them with the occurrence of mastitis in dairy cattle herds. Few correlations were observed among mastitis cases in dairy cattle herds and the raw-milk indicators of hygienic-sanitary quality. We observed a negative correlation between the log10 of mesophilic aerobic plate counts and psychotropic aerobic plate counts when compared with the occurrence of no bacterial growth. The log10 of thermophilic aerobic plate counts and yeasts and mould aerobic plate counts presented a positive correlation with the cases of infectious mastitis and mastitis caused by Staphylococcus spp.     

Growth responses of environmental mastitis pathogens to long-chain fatty acids

Streptococcus uberis, Streptococcus faecalis, Escherichia coli, and Klebsiella pneumoniae were tested for susceptibility to long-chain fatty acids predominant in teat canal keratin. Antibacterial activity of free fatty acids on each bacterial species was measured after 12 h in synthetic media. Growth responses of all three strains of Streptococcus uberis were completely inhibited by C18:3 and those of two of three strains by C18:2 at 1 micrograms/ml. None of the fatty acids tested were bactericidal to Streptococcus faecalis. Saturated fatty acids C14 and C16 were more bacteriostatic to Streptococcus faecalis than were polyene fatty acids. Growth responses of coliform species were not affected by long-chain fatty acids. In general, environmental mastitis pathogens were resistant to fatty acids predominant in teat canal keratin.     

Effects of Corynebacterium bovis infection on susceptibility to major mastitis pathogens

Experimental challenge procedures were used to study infectivity and virulence of Corynebacterium bovis. Challenge procedures using Staphylococcus aureus (Newbould 305) and Streptococcus agalactiae (McDonald 44) were used to study effects of Corynebacterium bovis infections on superinfection with major pathogens. Rate of infection under experimental challenge conditions was significantly greater with Corynebacterium bovis than previously observed for Staphylococcus aureus or Streptococcus agalactiae. Principal location of Corynebacterium bovis colonization appeared to the teat canal region, although the organism was isolated from 75% of the teat cisterns by puncture technique. Quarters with Corynebacterium bovis infections were more resistant to infection by Staphylococcus aureus than bacteriologically negative quarters. Quarters infected with Corynebacterium bacterium bovis were approximately 8.5-fold more susceptible to Streptococcus agalactiae infection than negative quarters. Somatic cell counts were doubled in negative quarters that developed Corynebacterium bovis infections; the geometric mean was 2.4 X 10(5).     

Climatic effects on occurrence of clinical mastitis

Clinical mastitis records for 6.5 yr from a large north Florida dairy and corresponding daily weather data were analyzed. Monthly incidence of clinical mastitis was expressed as percent of cow-days in milk and graphed against monthly average daily maximum temperature humidity index values and monthly total rainfall. No trends were evident with rainfall. In 3 of 6 yr, monthly incidence of clinical mastitis increased more than 50% above annual incidence, and this followed high monthly temperature-humidity values. Least squares was used to estimate regression coefficients of temperature-humidity index categories based on 999,969 Holstein records. A temperature humidity index category represented the number of days used to calculate average daily maximum temperature-humidity index value. Sources of variation in observed occurrence of clinical mastitis were cow, parity, month, year, interaction of parity by month, and continuous effects of temperature-humidity index categories 2, 6, 15, 30, 60, each to third order and 60 by parity interaction. In all temperature-humidity index categories as the temperature-humidity index value increased, occurrence of clinical mastitis increased. When values rose from 55 to 80, twice as many for 2 d cows showed signs of clinical mastitis.     

Common mammary pathogens and factors in infection and mastitis

Inflammation of the mammary gland, commonly known as mastitis, is considered a complex disease in view of its complexities of etiology, pathogenesis, sequela, therapy, and related aspects. The disease of most concern is the one produced by pathogenic microorganisms. However, as a prerequisite the pathogen must not only enter the mammary gland but also be able to survive and multiply in numbers sufficient to produce pathogenic effects. The normal bovine mammary gland is protected from such attacks by several anatomic and biologic armamenta which must be overcome by the invading pathogens. Organisms involved in causation of mastitis vary in their habitat, virulence, and susceptibility to host barriers. Similarly cows vary in their resistance to microbial entry into the mammary gland and subsequent response to overcome the infection. Only a few organisms, however, need to survive in the milieu of the mammary gland to produce mastitis. This paper is a general discussion of these aspects concerning bovine mastitis in relation to common mammary pathogens.     

Relationship between teat-end callosity and occurrence of clinical mastitis

A longitudinal study in 15 herds, with a total of 2157 cows, was conducted to examine the relationship between teat-end callosity (TEC) and the incidence of clinical mastitis. During the 1.5-yr study period, clinical mastitis was diagnosed by the farmers based on clinical signs. Teat-end callosity was scored every month according to a teat-end callosity classification system, which discriminates between teat-end callosity thickness (TECT) and roughness (TECR). Differences in TECT between healthy and clinical mastitis quarters within infected cows were small but significant 3 mo before (0.13 higher), in the month during which the clinical mastitis occurred (0.08 higher), and in the following 2 mo (0.06 and 0.05 higher). To compare TECT and TECR between cows with and without clinical mastitis, 199 cows with clinical mastitis were paired with control cows based on herd, days in milk, and parity. Clinical mastitis cows had more TEC than their healthy herd mates, particularly when clinical mastitis occurred between the second and fifth months of lactation. Clinical Escherichia coli mastitis in the second or third month of lactation occurred in cows with less TEC than in cows with clinical mastitis caused by other pathogens. Clinical culture-negative, yeast, Klebsiella pneumoniae, and Enterobacter aerogenes mastitis cows had more TECT and TECR than other cows with clinical mastitis in the same month of lactation. Pointed teat ends had higher TECT and TECR than flat or inverted teat ends. Teat-end callosity thickness increased with a higher milk yield at peak production.     

Use of neural networks to detect minor and major pathogens that cause bovine mastitis

The objectives of this research were to test the potential of unsupervised (USNN) and supervised neural network (SNN) models for detecting major and minor mastitis pathogens based on changes in milk parameters. A data set of 4,852 quarter milk samples with records for milk parameters and bacteriological status was used to train and validate the models by classifying milk samples into 3 different bacteriological states: not infected, intramammary infection (IMI) by minor pathogens, and IMI by major pathogens. Sensitivity of the USNN model was 97% for detecting noninfected quarters, 89% for minor pathogen IMI, and 80% for major pathogen IMI. Specificities of USNN models were close to 99% for all bacteriological states. The sensitivity of SNN models was affected by the ratio of infected to noninfected cases in the data set. As the ratio of infected to healthy cases increased from 1:1 to 1:10, detection accuracy for noninfected quarters increased from 82 to 98% but that for minor pathogen IMI decreased from 86 to 44%. The sensitivity for major pathogen IMI was 20% when the ratio was 1:1, but ranged from 20 to 40% when different ratios were tested. The SNN models indicated that somatic cell score and electrical resistance index had the most discriminating power. It was concluded that both USNN and SNN models were able to effectively differentiate between noninfected quarters and those infected by minor mastitis pathogens, and that the USNN model had a better agreement with results obtained from conventional microbiological methods. These types of models can be used in in-line milking systems to detect the infection status of a quarter and provide the farmer with diagnosing options for managing mastitis.     

Leukocytes--second line of defense against invading mastitis pathogens.

In mammals, neutrophile polymorphonuclear leukocytes constitute one of the essential body defenses against disease. In a large mammal, such as the dairy cow, billions of neutrophils are mobilized to fight infection. For example, over 50 million neutrophils per milliliter milk are commonly in a mammary quarter inflicted with clinical mastitis. However, in spite of these numerous leukocytes, pathogenic organisms remain viable. Recent evidence indicates that bacteria are not eliminated from a diseased quarter because the phagocytic capacity of the neutrophils is reduced in the mammary gland. The morphology and physiology of the leukocyte is examined in this review in an attempt to explain why the phagocytic capacity of the neutrophil is reduced in the mammary gland of the bovine.     

Growth of environmental mastitis pathogens in various bedding materials

The objective of the study was to determine whether, under controlled conditions, bedding materials vary in their ability to support growth of different environmental pathogens independent of the presence of feces, urine, or other contamination. Five sterilized bedding materials (fine hardwood chips, recycled dried manure, chopped newspaper, softwood sawdust, and chopped straw) and three bacterial species (Escherichia coli, Klebsiella pneumoniae, and Streptococcus uberis) were used for a total of 15 bedding/bacteria combinations, replicated in three trials. Samples were incubated at 37 degrees C, and bacterial counts were determined over 5 d. Rapid growth was seen in straw and recycled manure, some growth occurred in hardwood chips, and a rapid decline in bacterial counts was observed in paper and softwood sawdust. In general, K. pneumoniae and E. coli showed more rapid growth or less rapid decline than did S. uberis. These results demonstrate that clean, damp bedding may support bacterial growth and suggest that high bacterial counts under barn conditions are influenced by factors more complex than type of bedding used.     

Mammary gland expression of antibacterial peptide genes to inhibit bacterial pathogens causing mastitis

As a step toward prevention of bovine mastitis, a plasmid-mediated gene transfer technique was used to enable mammary cells to synthesize and secrete bovine lactoferricin and bovine tracheal antibacterial peptides. For this purpose, a series of mammary tissue-specific expression vectors, harboring the antibacterial peptide gene, the 5′-flanking regulation sequence of goat β-casein, and the bovine growth hormone polyadenylation signal sequence, were constructed using a eukaryotic expression vector pIRES1-neo. The mammary gland tissue-specific expression vector carrying the antimicrobial peptide genes dissolved in physiologic saline was injected directly into the lactating mammary glands of goats. The milk samples after injection were checked by Tricine-SDS-PAGE and bacterium inhibition zone assay. The results of these tests showed that the mammary gland tissue-specific expression vector driven by the goat β-casein gene promoter could efficiently direct the expression of antibacterial peptides in goat milk; the expression of antibacterial proteins lasted for 3 to 6 d. All of the milk samples collected from the mammary glands that had been injected with different vectors harboring the antibacterial peptide gene(s) exhibited bacteriostatic activity against different bacterial pathogens. These results demonstrated that the mammary gland tissue-specific expression vector could be used to introduce antibacterial peptide gene into the goat mammary gland, enabling secretion of a bioactive form of antibacterial peptide in the milk. This successful expression of antibacterial peptides in goat mammary glands provided a possible method to prevent mastitis in ruminants.     

Prevalence of mastitis pathogens in Ragusa, Sicily, from 2000 to 2006

The objective of this study was to report the prevalence of intramammary infections (IMI) in Ragusa, Sicily, from milk samples (n = 18,711) collected between October 2000 and June 2006 from 101 dairy herds. Milk samples were collected at 9,747 cow sampling events from 5,285 individual cows. Samples were individual quarter (92.8%) or composite samples (7.2%) from an individual cow. Logistic regression was used to examine the prevalence of IMI at the level of milk sample and at the level of cow, controlling for year and season of collection, type of sample (individual quarter or composite), and type of housing and bedding of the cow at the time of collection. Bedding and housing types were as follows, respectively (number of herd groups): bedding: none (61), organic [51 (straw, 50; sawdust, 1)], and sand (3); housing: bedded pack (37), free stalls (57), tie stalls (4), and paddock (17). Raw prevalence of cow IMI for a sample event was as follows (percentage of cow samples): no growth, 47.4%; coagulase-negative staphylococci, 22.6%; Staphylococcus aureus, 20.6%; other Streptococcus spp., 11.1%; Streptococcus agalactiae, 2.3%; coliform bacteria, 2.9%; and other organisms, 5.8%. Prevalence of IMI differed by bedding type for Staph. aureus (none, 24.5%; organic, 12.7%; sand, 12.3%) and coagulase-negative staphylococci (none, 13.1%; organic, 27.4%; sand, 26.9%) but not for Streptococcus spp. or coliform bacteria. Prevalence of Streptococcus spp. IMI differed by housing type (tie stalls, 22.2%; bedded pack, 12.8%; free stalls, 8.4%; paddock, 7.1%). Housing was not associated with the prevalence of IMI for other bacteria. Herd monthly prevalence of Staph. aureus and Streptococcus spp. IMI was associated with decreased mean milk production (Staph. aureus, −1.42 kg/d per cow, SEM 0.51; Streptococcus spp., −1.31 kg/d per cow, SEM 0.64) and increased mean linear score (Staph. aureus, 1.01 units/d per cow, SEM 0.16; Streptococcus spp., 0.59 units/d per cow, SEM 0.22). Herds (n = 11) with a mean linear score (MLS) less than 3.3 units had the lowest prevalence of Staph. aureus IMI and monthly MLS and the greatest monthly mean milk production compared with other herds grouped by MLS [MLS 3.31 to 3.99 (n = 20), MLS 4.00 to 4.46 (n = 20), MLS >4.46 (n = 17), and MLS not available (n = 33)]. Implementation of a milk quality program to control gram-positive organisms is important for Ragusa.     

Elimination of selected mastitis pathogens during the dry period

We aimed to evaluate the elimination of 4 different mastitis pathogens, Streptococcus agalactiae, Mycoplasma bovis, Staphylococcus aureus, and Streptococcus uberis, from infected udder quarters during the dry period using quantitative PCR. The second purpose of this study was to evaluate the association between milk haptoglobin (Hp) concentration and the presence of udder pathogens (Strep. agalactiae, Staph. aureus, M. bovis, and Strep. uberis) in udder quarter milk samples before and after dry period. Aseptic udder quarter milk samples (n = 1,001) were collected from 133 dairy cows at dry off and at the first milking after calving from 1 large dairy herd. Bacterial DNA of Strep. agalactiae, Staph. aureus, Strep. uberis, and M. bovis in the udder quarter milk samples was identified with commercial quantitative PCR analysis Mastitis 4B (DNA Diagnostic A/S, Risskov, Denmark). Milk Hp concentration (mg/L) was measured from udder quarter milk samples. The elimination rates during the dry period for M. bovis, Staph. aureus, Strep. agalactiae, and Strep. uberis were 86.7, 93.6, 96.2, and 100.0%, respectively. The new IMI rate was 3.0% for M. bovis, 2.9% for Staph. aureus, 2.4% for Strep. agalactiae, and 3.1% for Strep. uberis. The milk Hp concentration was significantly higher in udder quarter milk samples with blood and in samples positive for Strep. agalactiae at dry off and for Staph. aureus postcalving. Elevated milk Hp concentration was not associated with the presence of M. bovis in the udder quarter milk samples. In conclusion, elimination of Staph. aureus, Strep. agalactiae, and Strep. uberis during the dry period was high; the elimination of M. bovis from infected udder quarters was lower, but probably spontaneous. Additionally, milk Hp concentration may be used as a marker for udder inflammation when combined with the bacteriological results at dry off and postpartum.     

Effective treatment of Streptococcus uberis clinical mastitis to minimize the use of antibiotics

Antibiotic regimens (intramammary antibiotic, penicillin-based parenteral treatment) and intramuscular oxytocin were tested for effectiveness against experimental infection by Streptococcus uberis with the following results from 54 animals: a) no treatment led to deterioration of infected quarters, requiring intervention within 48 h for cow health; b) aggressive intramammary antibiotic at every milking achieved 70% clinical cure in 3 d and 100% cure within 6 d; overall bacteriological cure was 80%; c) parenteral treatment alone used about 14 times as much antibiotic with 18% clinical cure in 3 d and 91% within 6 d; overall bacteriological cure was 80%; d) combination of aggressive intramammary and parenteral treatments achieved 61% clinical cure in 3 d and 100% within 6 d; overall bacteriological cure was 72%; e) intramammary antibiotic at labeled rates (1x for 3 d) achieved 27% clinical cure in 3 d but 91% within 6 d of treatment; overall bacteriological cure was 64%; f) use of oxytocin alone for 3 d failed to achieve clinical improvement with an increase in the severity of mastitis; g) combining oxytocin with labeled use of intramammary antibiotic (1x for 3 d) was unsuccessful: 0% clinical cures in 3 d, 10% in 6 d; significantly poorer than intramammary antibiotic alone. Extended treatment periods with parenteral or intramammary antibiotics resulted in positive inhibitory tests for milk from individual quarters up to 8 d after treatment. Aggressive intramammary antibiotic was the most effective treatment for fastest cure clinically and bacteriologically using least antibiotic.     

Dye marked antibiotics for lactating cow mastitis therapy

Dye markings of intramammary antibiotic infusions could give a dairy farmer immediate visual warning that milk contains antibiotic residue. Six dye and antibiotic preparations for lactating cows were studied for rates of dye and antibiotic milk-out. Albacillin containing 1 X 10(5) IU of penicillin plus 150 mg of novobiocin combined with 25, 125, or 250 mg of Food, Drug, and Cosmetic Blue No. 1 or No. 2 per infusion was used. Thirty cows with healthy udders producing 13.6 to 22.7 kg milk per day were treated in all quarters twice in 24 h (0500 and 1500 h). Milk samples from 14 posttreatment twice per day milkings (0500 and 1500 h) were tested for dye and antibiotic residue. Dye content was determined by a visual method and subvisually by an ion exchange resin method. No antibiotic residues were found by the cylinder plate method after the second to fourth posttreatment milkings. Antibiotic residue was detected up to the sixth milking by Delvotest-P. Depending on the dye type and its concentration, milk was visibly blue for one to four milkings. Subvisual quantities for dye were detectable by the ion exchange resin method for three to five milkings. The preparation showing the most promise for farm use contained 250 mg Blue No. 1 per infusion. Milk from cows treated with this preparation contained visually and subvisually detectable dye through three or four and five milkings, respectively. The dye persistence exceeded or coincided with the maximum antibiotic persistence in nearly all cows treated regardless of dye formulation or method of antibiotic detection.     

Dye marked antibiotics in dry cow mastitis therapy

Eight dry cow dye/antibiotic preparations were used to determine dye and antibiotic excretion endpoints. Formulations contained 400 mg of novobiocin with 25 or 250 mg Food Drug and Cosmetic Blue No. 1 per infusion or 400 mg novobiocin plus 200,000 IU penicillin with 25, 125, or 250 mg Blue No. 1 or 2 dye per infusion. Fifty-two cows entering their second or later lactations were treated in two or four quarters. Milk was sampled postpartum and tested for residues. Dye content was determined visually and subvisually. Antibiotic content was analyzed by the cylinder plate, or Delvotest-P, or by both methods. One sample contained penicillin residues. Three samples had visual dye content of Blue No. 1. Blue No. 2 was not detected. Subvisual dye was detected for one to four milkings in most cows treated with preparations containing Blue No. 1. Cows were treated within 1 to 7 d prepartum with novobiocin plus penicillin containing 250 mg Blue No. 1. Visual dye persisted from 3 to 15 milkings postpartum and subvisual dye for one to two milkings longer. Penicillin was present in postpartum milk in five of seven cows. Novobiocin persisted in three of seven cows.     

Detection of mastitis pathogens by analysis of volatile bacterial metabolites

The ability to detect mastitis pathogens based on their volatile metabolites was studied. Milk samples from cows with clinical mastitis, caused by Staphylococcus aureus, coagulase-negative staphylococci, Streptococcus uberis, Streptococcus dysgalactiae, and Escherichia coli were collected. In addition, samples from cows without clinical mastitis and with low somatic cell count (SCC) were collected for comparison. All mastitis samples were examined by using classical microbiological methods, followed by headspace analysis for volatile metabolites. Milk from culture-negative samples contained a lower number and amount of volatile components compared with cows with clinical mastitis. Because of variability between samples within a group, comparisons between pathogens were not sufficient for classification of the samples by univariate statistics. Therefore, an artificial neural network was trained to classify the pathogen in the milk samples based on the bacterial metabolites. The trained network differentiated milk from uninfected and infected quarters very well. When comparing pathogens, Staph. aureus produced a very different pattern of volatile metabolites compared with the other samples. Samples with coagulase-negative staphylococci and E. coli had enough dissimilarity with the other pathogens, making it possible to separate these 2 pathogens from each other and from the other samples. The 2 streptococcus species did not show significant differences between each other but could be identified as a different group from the other pathogens. Five groups can thus be identified based on the volatile bacterial metabolites: Staph. aureus, coagulase-negative staphylococci, streptococci (Strep. uberis and Strep. dysgalactiae as one group), E. coli, and uninfected quarters.     

Antimicrobial activity of crude extracts from actinomycetes against mastitis pathogens

The emergence of antimicrobial resistance to commonly used antibiotics has necessitated the development of new antimicrobial products. Crude extracts produced by actinomycetes contain antimicrobial metabolites that can inhibit bacterial growth. The objective of our study was to evaluate the antimicrobial activity of crude extracts (Caat1–54 and CaatP5–8) produced by actinomycetes against isolates of Staphylococcus aureus, Staphylococcus chromogenes, Streptococcus dysgalactiae, and Streptococcus uberis, which were obtained from the milk of cows affected by mastitis in 23 dairy herds. Twenty isolates of each bacterial species were used to define minimum inhibitory concentrations (MIC) of both crude extracts and ceftiofur (positive control). The MIC₅₀ and MIC₉₀ were defined at the concentration required to inhibit the growth of 50 and 90% of bacterial isolates tested, respectively. The MIC results were evaluated by survival analysis. Staphylococcus aureus isolates presented MIC₉₀ of Caat 1–54 ≥6.25 µg/mL, ceftiofur ≥12.5 µg/mL, and Caat P5–8 ≥100 µg/mL. Streptococcus uberis presented MIC₉₀ of ceftiofur ≥0.39 µg/mL, Caat 1–54 ≥50 µg/mL, and Caat P5–8 ≥100 µg/mL. Staphylococcus chromogenes isolated from subclinical mastitis presented MIC₉₀ of Caat 1–54 ≥0.78 µg/mL and ceftiofur and Caat P5–8 of ≥6.25 and ≥100 µg/mL, respectively. Streptococcus dysgalactiae isolated from clinical mastitis presented similar MIC₉₀ values between antimicrobials tested (ceftiofur, Caat 1–54, and Caat P-58), but these values (≥100 µg/mL) were higher than the values obtained from other pathogens evaluated in the present study. Our results indicate that Caat 1–54 and Caat P5–8 crude extracts present in vitro antimicrobial activity against isolates of Staph. aureus, Staph. chromogenes, Strep. dysgalactiae, and Strep. uberis isolated from clinical and subclinical mastitis.     

Antibacterial effect of caprylic acid and monocaprylin on major bacterial mastitis pathogens

Bovine mastitis is the most significant economic drain on the worldwide dairy industry. Concerns regarding poor cure rates, emergence of bacterial resistance, and residues in milk necessitate development of alternative therapeutic approaches to antibiotics for treatment of mastitis. A variety of free fatty acids and their monoglycerides have been reported to exert antimicrobial activity against a wide range of microorganisms. The objective of our study was to examine the efficacy of caprylic acid, a short-chain fatty acid, and its monoglyceride, monocaprylin, to inactivate common mastitis pathogens, including Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, Staphylococcus aureus, and Escherichia coli. Milk samples containing 50 mM or 100 mM caprylic acid, and 25 mM or 50 mM monocaprylin were inoculated separately with a 3-isolate mixture of each of the 5 pathogens, and incubated at 39°C. Populations of surviving bacteria were determined at 0 min, 1 min, 6 h, 12 h, and 24 h of incubation. Both caprylic acid and monocaprylin reduced all 5 pathogens by >5.0 log cfu/mL after 6 h of incubation. Among the bacterial species tested, Strep. agalactiae, Strep. dysgalactiae, and Strep. uberis were most sensitive, and E. coli was most tolerant to caprylic acid and monocaprylin. Results of this study indicate that caprylic acid and monocaprylin should be evaluated as alternatives or adjuncts to antibiotics as intra-mammary infusion to treat bovine mastitis.     

Relationship between antimicrobial drug usage and antimicrobial susceptibility of gram-positive mastitis pathogens

The objective of this study was to analyze relationships between usage of antimicrobial drugs on dairy farms and results of antimicrobial susceptibility testing of mastitis pathogens. Exposure to selected antimicrobial drugs (n = 10) was standardized by calculation of the number of defined daily doses used per cow. Farms (n = 40) were categorized based on amount of antimicrobial exposure: organic (no usage); conventional-low usage (conventional farms not using or using less than or equal to the first quartile of use of each compound); and conventional-high usage (conventional farms using more than the first quartile of a particular compound). The minimum inhibitory concentration (MIC) of selected antimicrobial drugs was determined using a commercial microbroth dilution system for isolates of Staphylococcus aureus (n = 137), coagulase-negative staphylococci (CNS, n = 294), and Streptococcus spp. (n = 95) obtained from subclinical mastitis infections. Most isolates were inhibited at the lowest dilution tested of most antimicrobial drugs. Survival curves for Staph. aureus and CNS demonstrated heterogeneity in MIC based on the amount of exposure to penicillin and pirlimycin. For CNS, farm type was associated with the MIC of ampicillin and tetracycline. For Streptococcus spp., farm type was associated with MIC of pirlimycin and tetracycline. For all mastitis pathogens studied, the MIC of pirlimycin increased with increasing exposure to defined daily doses of pirlimycin. The level of exposure to most other antimicrobial drugs was not associated with MIC of mastitis pathogens. A dose-response effect between antimicrobial exposure and susceptibility was observed for some pathogen-antimicrobial combinations, but exposure to other antimicrobial drugs commonly used for prevention and treatment of mastitis was not associated with resistance.