Effect of lactoferrin in combination with penicillin on the morphology and the physiology of Staphylococcus aureus isolated from bovine mastitis

The objective of the present study was to evaluate the therapeutic potential of bovine lactoferrin or lactoferricin in combination with penicillin G against Staphylococcus aureus. Minimal inhibitory concentrations of lactoferrin, lactoferricin, penicillin, and combinations of lactoferrin or lactoferricin with penicillin were determined for 15 S. aureus strains including several strains resistant to beta-lactam antibiotics. The fractional inhibitory concentration index indicated a synergistic effect between lactoferrin and penicillin. Combination of lactoferrin with penicillin increased the inhibitory activity of penicillin by two- to fourfold and reduced the growth rate in S. aureus strains tested, whereas the increase in the inhibitory activity of lactoferrin by penicillin was 16- to 64-fold. The addition of iron to the medium containing a combination of penicillin and lactoferrin had no effect on growth inhibition. Electron microscopy revealed that concentration below the minimal inhibitory concentrations of penicillin induced important ultrastructure alterations, which were further enhanced by the presence of lactoferrin. When S. aureus cells were grown in the presence of a combination of penicillin and lactoferrin, changes in the protein profile of the bacteria, including the disappearance of several protein bands due to the presence of lactoferrin, were observed. These data suggest that bovine lactoferrin or lactoferricin in combination with beta-lactam antibiotics can increase the antibacterial activity of these antibiotics against S. aureus resistant to antibiotics.     

Antimicrobial susceptibility of Staphylococcus aureus from retail ground meats

The aim of this study was to characterize antimicrobial resistance in Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA), recovered from raw retail meat products purchased in the Washington, D.C., area. From March to August 2008, 694 samples of ground beef (n = 198), ground pork (n = 300), and ground turkey (n = 196) were collected by random sampling from stores of three grocery chains. In total, 200 S. aureus isolates (29%) were recovered by direct plating. When tested for susceptibility to 22 antimicrobials, 69% of the S. aureus isolates were resistant to tetracycline, 26% to penicillin, 17% to ampicillin, 13% to methicillin, 8% to erythromycin, 4.5% to clindamycin, 1.5% to gentamicin, and 0.5% to chloramphenicol, oxacillin, cefoxitin, or quinupristin-dalfopristin. However, 27% of the isolates were susceptible to all tested antimicrobials. More turkey and pork isolates were resistant to ampicillin, penicillin, and tetracycline than were beef isolates (P < 0.05). Additionally, 17% of the turkey and 17% of the pork isolates were resistant to methicillin (MIC ≥ 16 μg/ml), whereas no beef isolates were resistant to the antimicrobial agent. A single MRSA (methicillin MIC > 32 μg/ml) isolate containing the mecA gene with additional resistance to erythromycin, clindamycin, oxacillin plus 2% NaCl, cefoxitin, ampicillin, penicillin, quinupristin-dalfopristin, tetracycline, and gentamicin was recovered from one pork sample. The presence of antimicrobial-resistant S. aureus, coupled with the relative lack of such studies in the United States, suggests that further investigations on MRSA in the food supply are needed despite the low rate of MRSA found in this particular study.     

Antimicrobial susceptibility and coagulase gene typing of Staphylococcus aureus isolated from bovine clinical mastitis cases in Turkey

The objectives of this study were to determine antimicrobial resistance patterns of Staphylococcus aureus strains isolated from bovine clinical mastitis cases and to subtype the strains by polymerase chain reaction (PCR) technique based on coagulase gene polymorphism. Two hundred sixty-five S. aureus isolates collected from individual animals in different herds (n = 235) from 1995 to 2004 were tested for susceptibility to penicillin, ampicillin, amoxicillin-clavulanate, oxacillin, oxytetracycline, enrofloxacin, kanamycin-cephalexin, and trimethoprim-sulfamethoxazole using the agar disc diffusion test. Strains were also tested for β-lactamase production. A total of 29.8% of the strains were susceptible to all antimicrobial agents tested. The highest resistance was observed in 63.3% of the strains against β-lactam antibiotics, penicillin and ampicillin. Oxytetracycline resistance was observed in 27.9% of the strains, either alone or in combination with β-lactams. No resistance was detected for amoxicillin-clavulanate, oxacillin, enrofloxacin and kanamycin-cephalexin. β-Lactamase production and resistance to β-lactam antibiotics were usually correlated. Resistance against β-lactams increased from 43.5% in 1995 to 58 to 77% from 1999 to 2004.One hundred twenty-five strains were examined for coagulase gene polymorphism. The isolates were subtyped into 4 types by coagulase gene-based PCR. A predominant 1000-bp PCR product was observed in 60.8% of the isolates typed. The results indicate that a few coagulase gene types of S. aureus are responsible for the majority of bovine clinical mastitis cases in one province of Central Anatolia region, Turkey.     

Antibiotic resistance of coagulase-negative staphylococci associated with food and used in starter cultures

The resistance of 330 coagulase-negative staphylococci (CNS) associated with food or used in starter cultures and belonging to the species Staphylococcus carnosus, Staphylococcus condimenti, Staphylococcus piscifermentans, Staphylococcus equorum, Staphylococcus succinus and Staphylococcus xylosus, against 21 antibiotics was determined using the disk diffusion method. The incidence and number of resistances was found to be species and source of isolation dependent. Most strains of S. equorum (63%), S. succinus (90%) and S. xylosus (95%) exhibited resistances against up to seven antibiotics, whereas only few strains of S. carnosus (12%) and S. piscifermentans (27%) were antibiotic resistant. Resistances to lincomycin, penicillin, fusidic acid, oxacillin, ampicillin and tetracycline were predominant. Among strains of S. xylosus, the incidence of resistance ranged from 22% for tetracycline up to 69% for penicillin. Concerning the source of isolation, resistances were often determined in strains of S. equorum, S. succinus and S. xylosus isolated from cheese (87%) and sausage (83%), and strains of S. xylosus obtained from meat starter cultures (93%). Remarkably, all CNS were sensitive to the clinically important antibiotics chloramphenicol, clindamycin, cotrimoxazol, gentamicin, kanamycin, linezolid, neomycin, streptomycin, synercid and vancomycin. The phenotypic resistances to ss-lactam antibiotics, lincomycin and tetracycline were verified by PCR amplification and could be traced back to the genes blaZ, lnuA and tetK, respectively. This study permitted a comprehensive insight into the incidence of antibiotic resistances in food-associated CNS.     

Antimicrobial susceptibility of Staphylococcus aureus isolated from bovine mastitis in Europe and the United States

Minimum inhibitory concentrations were determined for 811 strains of Staphylococcus aureus isolated from cases of bovine mastitis in 11 countries. The countries and number of isolates included Denmark (105), England (92), Finland (95), Germany (103), Iceland (22), Ireland (42), Norway (101), Sweden (123), Switzerland (69), United States (53), and Zimbabwe (6). The antimicrobial agents tested were penicillin, ampicillin, oxacillin, cephalothin, ceftiofur, amoxicillin + clavulanate, penicillin + novobiocin, enrofloxacin, premafloxacin, erythromycin, clindamycin, lincomycin, pirlimycin, neomycin, lincomycin + neomycin, and sulfamethazine. The MIC90 for these antimicrobial agents for all strains were 0.5, 1.0, 1.0, 0.5, 1.0, < or =0.06, 0.125, 0.125, < or =0.0078, 0.5, 1.0, 16.0, 1.0, 2.0, 0.5, and 4.0 microg/ml, respectively. Overall, only small variations between countries were seen in the MIC90 for the majority of compounds tested. Of the strains tested, 35.6% were positive for beta-lactamase production on initial testing, with an additional 21.3% positive after induction by penicillin. In conclusion, the overall level of resistance was generally low for all antimicrobial agents tested regardless of country. Given the differences in antimicrobial use in various countries, the widespread adoption of mastitis control programs to prevent infections limits the exposure of S. aureus infected animals to antimicrobial drugs.     

Short communication: antimicrobial drug susceptibility of Staphylococcus aureus from subclinical bovine mastitis in Italy

The antimicrobial susceptibility of 68 Staphylococcus aureus isolates collected during 2004 from milk of cows affected by subclinical mastitis was examined. The antimicrobial agents tested were the β-lactams, penicillin G, amoxicillin, ampicillin, cloxacillin, amoxicillin + clavulanate, cephalonium, and cefoperazone; and other drugs including lincomycin, oxytetracycline, doxycycline, and kanamycin. Minimum inhibitory concentrations recorded show that only certain β-lactamase-resistant penicillins (specifically cloxacillin) or penicillin combinations (amoxicillin + clavulanate) were consistently effective against Staph. aureus, whereas the other β-lactam derivatives and drugs from other pharmacological groups were either moderately effective or ineffective. Thus, β-lactamase-resistant penicillins are to be considered the antimicrobial agents of choice for treatment of bovine mastitis resulting from infection by Staph. aureus.     

广东地区46株食源性金黄色葡萄球菌耐药性、生物被膜形成能力及IcaAB基因的特征分析

目的 分析广东地区食源性金黄色葡萄球菌耐药现状、生物被膜形成能力及生物被膜相关基因IcaAB的特征。方法 对46株食源性金黄色葡萄球菌采用微量肉汤稀释法进行药敏实验,采用结晶紫法检测生物被膜生成能力,运用聚合酶链式反应(polymerase chain reaction, PCR)检测生物被膜相关基因。结果 97.83%的菌株对至少1种抗生素耐药, 30.43%为多重耐药菌株,对青霉素(97.83%)、氨苄西林(97.83%)耐药性最高; 65.22%菌株能够形成生物被膜,其中强、中和弱粘附生物被膜能力的菌株分别占17.39%、30.43%和17.39%,且产生物被膜能力强的菌株对特定抗生素(头孢西丁、环丙沙星)的耐药性更高(P<0.05); PCR结果显示, icaA、icaB的检出率分别为56.67%、36.67%,基因携带率与产膜能力无显著关联(P>0.05)。结论 广东地区食源性金黄色葡萄球菌普遍表现出耐药性和生物被膜形成能力,对公共卫生带来挑战,需采用措施以确保公共安全。     

苏州市食源性金黄色葡萄球菌耐药分析及基因分型

目的 对苏州市2014~2017年分离的食源性金黄色葡萄球菌进行耐药谱和分子分型分析, 初步建立PFGE(pulsed-field gel electrophoresis)分型数据库。方法 采用微量肉汤稀释法检测金黄色葡萄球菌(Staphylococcus aureus, SAU)对19种常用的抗生素的耐药性, 采用Sma I进行酶切, 脉冲场凝胶电泳进行分子分型。菌株指纹图谱用BioNumerics v6.6软件进行分析。结果 81株SAU中检出10株耐甲氧西林金黄色葡萄球菌(methicillin-resistance Staphylococcus aureus, MRSA), 检出率12.3%。有72株菌存在不同程度的耐药, 耐药率达到88.9%, 其中对青霉素的耐药率最高(82.7%), 其次是氨苄西林(66.7%), 其后是克林霉素和红霉素(均为50.6%)。耐受3种和3种以上药物的多重耐药菌株共38株, 多重耐药率达46.9%。MRSA的多重耐药率高于甲氧西林敏感型金黄色葡萄球菌(methicillin-sensitive Staphylococcus aureus, MSSA), 差异具有统计学意义(P<0.05)。除8株菌不可被PFGE分型外, 其余73株菌可以被分为37个不同型别, 其中有4个优势型别共27株菌, 在可分型菌株中占比达37.0%。这些同源菌株分离自不同时间和地点, 有些还存在一定程度的变异。结论 食源性金黄色葡萄球菌耐药现象普遍存在, 多重耐药率较高, PFGE型别较为多样但有优势型别存在, 耐药谱和PFGE型别关联性较差。     

Phenotypic and genetic antibiogram of methicillin-resistant staphylococci isolated from bovine mastitis in Korea

Staphylococcus aureus belongs to the group of major contagious mastitis pathogens, whereas the coagulase-negative staphylococci (CNS) are also capable of causing opportunistic bovine mastitis. Many of these strains are resistant to penicillin or ampicillin because of the long-term use of β-lactam antibiotics in agricultural and healthcare settings. Based on the simple and highly specific coagulase genotyping by PCR-RFLP used for discriminating among Staph. aureus strains, the relationship between phenotypic antibiogram and the polymorphism of coagulase gene was determined in this study. The staphylococci strains (835 Staph. aureus and 763 CNS) were isolated from 3,047 bovine mastitic milk samples from 153 dairy farms in 8 provinces from 1997 to 2004 in the Republic of Korea. Twenty-one (2.5%) Staph. aureus and 19 (2.4%) CNS strains were resistant to methicillin [oxacillin minimum inhibitory concentration (MIC) ≥4 μg/mL]. The mecA gene was also found in 13 methicillin-resistant Staph. aureus (MRSA) and 12 methicillin-resistant CNS (MRCNS) isolates with a significantly higher detection rate of the mecA gene in MRSA with high MIC (≥16 μg/mL) compared with those with MIC ≤ 8 μg/mL. Methicillin-resistant Staph. aureus and MRCNS were also more resistant to other antibiotics (ampicillin, cephalothin, kanamycin, and gentamicin) than methicillin-susceptible staphylococci. Among 10 different coa PCR-RFLP patterns (A to J) in 706 Staph. aureus strains, the main types were A (26.9%), B (17.0%), G (10.5%), and H (15.4%), with the frequent observation of the A and H types (6 and 10 isolates) in MRSA. This study indicates that major epidemic Staph. aureus clones may be spread between different dairy farms, and the profile of coa genotype can be applied for epidemiological investigations and control of bovine mastitis, particularly one caused by MRSA with specific prevalent coa types.     

Antibiotic susceptibility patterns for environmental streptococci isolated from bovine mastitis in central California dairies

Environmental streptococci are frequently isolated from bovine mastitis in dairy cows with only limited information available on the antimicrobial susceptibility of these organisms. A total of 362 environmental streptococci isolated from cases of bovine mastitis from the central San Joaquin Valley of California over a 3-yr period were used in the study. Overall, 39.9% of the strains tested were Streptococcus uberis, 42.2% were Streptococcus dysgalactiae, and 11.1% were Enterococcus spp. The antimicrobial susceptibility for these organisms was determined for the following antimicrobial agents: penicillin, ampicillin, cephalothin, ceftiofur, penicillin + novobiocin, erythromycin, pirlimycin, tetracycline, and sulfadimethoxine. Results demonstrate substantial differences in the susceptibility patterns for the various organisms collectively referred to as the environmental streptococci. The MIC90 for penicillin was 0.06 microg/ml for 152 strains of S. dysgalactiae compared with 0.25 microg/ml for 133 strains of S. uberis. However, the Enterococcus spp. were the most resistant organisms tested. These data also indicate that the use of interpretive criteria based on human data may provide misleading results. In conclusion, these data confirm that the environmental streptococci are a diverse group of organisms comprised of several different genera and species and that identification of environmental streptococci to the species level is needed to appropriately modify control methods. Moreover, the use of the agar disk diffusion (Kirby-Bauer) susceptibility test for agents with human-based interpretive criteria is contraindicated, and these tests should only be performed with agents with mastitis specific interpretive criteria.     

2015~2018年北京市食源性金黄色葡萄球菌耐药性分析

目的 掌握北京市食源性金黄色葡萄球菌对常用抗生素的耐药谱和耐药趋势。方法 收集2015?2018年北京市食源性致病菌监测网分离到的金黄色葡萄球菌103株, 采用CLSI推荐的微量肉汤稀释法检测金黄色葡萄球菌对11种常用抗生素的敏感性。结果 103株菌中有102株表现为对至少1种抗生素耐药, 耐药率排在前三位的是青霉素、红霉素和克林霉素, 其中对青霉素的耐药率最高, 为99.0%。检出耐甲氧西林金黄色葡萄球菌8株, 占7.8%, 未见万古霉素耐药株。45.6%(47/103)的菌株同时耐受3类和3类以上抗生素, 表现为多重耐药, 同时耐受抗生素种类最高为7类。共存在26种耐药谱, 优势耐药谱为青霉素、青霉素-红霉素-克林霉素-复方新诺明-庆大霉素和青霉素-红霉素。结论 2015~2018年北京市食源性金黄色葡萄球菌整体耐药水平较高, 且多重耐药情况严重, 并出现MRSA株, 应继续加强北京市食源性金黄色葡萄球菌的耐药性监测。     

Impact of environmental stress desiccation, acidity, alkalinity, heat or cold on antibiotic susceptibility of Cronobacter sakazakii

Cronobacter sakazakii is an emerging foodborne pathogen that has been implicated in severe forms of meningitis, septicemia or necrotizing colitis in pre-term neonates. Although illness outbreaks (primarily associated with powdered infant formula, PIF) caused by this pathogen are rare, the case-fatality rate may reach 50%. Successful treatment of C. sakazakii infection is reliant upon clinical use of antibiotics (AB) such as ampicillin. Recent reports showed increased resistance of C. sakazakii to broad-spectrum antibiotics. The objective of this study was to evaluate the effect of extreme pH (3.5 for 30 min or 11.25 for 5 min), cold (4 °C for 24 h), heat (55 °C for 5 min), and desiccation (cells were dried at 40 °C for 2 h and held at 21 °C for 4 d) stresses on susceptibility of five isolated strains of C. sakazakii to streptomycin, gentamicin, kanamycin, neomycin, tetracycline, doxycycline, tilmicosin, florfenicol, ampicillin, amoxicillin, vancomycin, ciprofloxacin and enrofloxacin. All unstressed strains of C. sakazakii were sensitive to streptomycin, gentamycin, kanamycin, ciprofloxacin, enrofloxacin, ampicillin and amoxicillin, but were moderately resistant or resistant to the rest. Exposing cells to alkaline or acidic stress did not change their sensitivity toward streptomycin, gentamycin, kanamycin or ciprofloxacin, but their resistance toward the other AB was increased. Cells stressed by desiccation showed increased sensitivity toward streptomycin, gentamicin, kanamycin, ciprofloxacin, enrofloxacin, ampicillin and doxycycline, but showed resistance toward the others. Cold-stressed cells were more sensitive to streptomycin, gentamicin, kanamycin, and ciprofloxacin compared with heat-stressed cells, but both heat and cold-stressed cells showed increased resistance toward all the other AB. Results obtained will help in understanding the effect of environmental stresses during processing on C. sakazakii susceptibility to AB.     

Comparison of antibiogram, staphylococcal enterotoxin productivity, and coagulase genotypes among Staphylococcus aureus isolated from animal and vegetable sources in Korea

Staphylococcal food poisoning is caused by enterotoxin-producing Staphylococcus aureus. We investigated the prevalence of such organisms in samples of bovine mastitic milk (n = 714), raw meat (n = 139), and vegetables (n = 616). We determined the degrees of relatedness of isolates as indicated by antibiogram, staphylococcal enterotoxin (SE) productivity, and coagulase gene restriction fragment length polymorphism analysis. We examined 297 S. aureus isolates and found SE production in 57 (31.8%), 4 (7.8%), and 49 (73.1%) isolates from raw milk, raw meat, and vegetables, respectively. A high proportion of the isolates obtained from milk produced more than two types of toxins (mainly SEA, SEB, and/or SEC), whereas isolates from raw meat and vegetables primarily produced SEA alone. Most isolates were sensitive to cephalothin (97.6%), gentamicin (80.8%), erythromycin (79.5%), and tetracycline (72.7%), but were resistant to penicillin (90.2%) and ampicillin (88.9%). The proportion of antibiotic-resistant isolates differed according the source of the bacteria; the milk and vegetable isolates were more resistant to penicillin and ampicillin than were the meat isolates (P < 0.05), whereas tetracycline resistance was limited to the milk and vegetables isolates. The coagulase genotypes (I to XII) varied with the source of the organism, and only a few genotypes prevailed in each source: II (42.4%) and IV (24%) types in isolates from milk, IX (35.3%) and XI (45%) from raw meat, and III (40.3%) and XII (32.8%) from vegetables. These findings suggest that remarkable differences exist in antibiogram, SE productivity, and coagulase genotypes, resulting in limited clonal transmission of S. aureus into various food sources. As enterotoxin production only occurs when S. aureus grows to high numbers, staphylococcal food poisoning can be prevented by proper refrigeration.     

Antimicrobial susceptibility of Staphylococcus aureus isolated from bovine mastitis in Argentina

A total of 206 strains of Staphylococcus aureus isolated from bovine clinical and subclinical mastitis in Argentina during 1996 to 1998 were investigated for their in vitro susceptibility to several antimicrobial agents. Minimum inhibitory concentrations that inhibit 90% of the strains tested reported in micrograms per milliliters were: 1.5, 0.5, 0.75, 1.50, 0.75, 1.0 and 0.125 for penicillin, oxacillin, cephalothin, gentamicin, erythromycin, clindamycin and ampicillin-sulbactam, respectively. Resistance was detected in 83 (40.3%), 24 (11.6%), 16 (7.7%) and 7 (3.4%) S. aureus isolates for penicillin, erythromycin, pirlimycin and gentamicin, respectively. No resistance was detected for oxacillin, cephalothin and ampicillin-sulbactam. Results indicated that S. aureus isolates in Argentina exhibited high resistance to penicillin of all antimicrobial agents tested.     

2016—2020年上海地区乳源金黄色葡萄球菌耐药性及最低抑菌浓度变迁情况分析

分析上海地区乳源金黄色葡萄球菌对常见抗菌药物的耐药性及其MIC变迁情况,加强致病菌耐药性检测。方法 采用微量肉汤稀释法对2016-2020年间采集的349株金黄色葡萄球菌进行10种常见抗菌药物敏感性测试,通过分析药敏数据了解5年间上海地区乳源金黄色葡萄球菌耐药性变迁的情况。结果 349株金黄色葡萄球菌对青霉素（74.2%）耐药率较高,对其余9种抗菌药物的耐药率较低,均在20%以下。五年间,上海地区乳源金黄色葡萄球菌对青霉素、苯唑西林及阿莫西林/克拉维酸的抑制50%微生物生长的抑菌浓度（minimal inhibitory concentration,MIC）MIC50值及抑制90%微生物生长的抑菌浓度MIC90值均呈下降趋势;对头孢西丁、头孢噻呋、恩诺沙星、氧氟沙星及庆大霉素的MIC50值五年来无明显变化,MIC90值呈下降趋势;对万古霉素和利奈唑胺的MIC50值和MIC90值五年来无明显变化,均处于较低水平。结论 金黄色葡萄球菌的耐药情况近年来有好转趋势,但仍需继续加强乳源金黄色葡萄球菌耐药性监测,为科学合理使用抗菌药物提供技术支撑。     

Minimum inhibitory concentrations of Staphylococcus aureus recovered from clinical and subclinical cases of bovine mastitis

Antimicrobials are often used for treatment of bovine mastitis and the possibility of selection for resistant bacteria must be considered. The objectives of this study were to determine the minimum inhibitory concentration of Staphylococcus aureus recovered from cases of clinical and subclinical bovine mastitis, and to determine the prevalence of multidrug resistance in this population. Milk samples were collected from cows on commercial dairy herds (n=13), including quarters (n=1,574) of cows with subclinical mastitis cases, and cows experiencing clinical mastitis cases (n=608). Selected Staph. aureus isolates, obtained from clinical (n=58) and subclinical (n=58) mastitis cases, were used to determine minimum inhibitory concentrations of 12 selected antimicrobials. Of Staph. aureus isolates tested, 87 (75%) did not exhibit resistance to any antimicrobial, 28 (24.1%) exhibited resistance to 1 (n=21) or 2 (n=7) classes of antimicrobials, and 1 (0.9%) exhibited multidrug resistance. All Staph. aureus (clinical and subclinical cases) were inhibited by the range of concentrations tested for ceftiofur and oxacillin. Moreover, no isolates obtained from clinical mastitis cases exhibited resistance to cephalothin, penicillin-novobiocin, or sulfadimethoxine. Of isolates, 3 exhibited resistance to enrofloxacin. Of isolates exhibiting resistance to more than 1 antimicrobial, independent of antimicrobial class, the combination of erythromycin and tetracycline, and ampicillin and penicillin accounted for the majority of resistance. Of isolates tested, 19% were resistant to tetracycline and 14% were resistant to penicillin. Survival curves of Staph. aureus relative to minimum inhibitory concentration demonstrated heterogeneity among case types for ceftiofur, cephalothin, and erythromycin. Multidrug resistance was identified in only 1 isolate obtained from a single farm.     

Short communication: N-Acetylcysteine–mediated augmentation of β-lactam antibacterial activity against methicillin-resistant Staphylococcus aureus isolated from bovine mastitis cases

The present study investigated the effects of N-acetylcysteine (NAC) on β-lactam antibacterial activity against 20 methicillin-resistant Staphylococcus aureus (MRSA) isolates from bovine mastitis. Minimum inhibitory concentrations (MIC) were determined by the E-test method. The presence of 10 mM NAC reduced the MIC of penicillin, ampicillin, oxacillin, cefoxitin, ceftazidime, and cefotaxime to MRSA. Importantly, the MIC of cefoxitin in MRSA in the presence of NAC was lower than the susceptible breakpoint of cefoxitin. The results provide a new way to use current β-lactam antibiotics combined with NAC against MRSA.     

Antimicrobial susceptibility of Listeria monocytogenes isolated from food and clinical cases in Navarra, Spain

The susceptibility of 440 Listeria monocytogenes strains isolated from food (n=401) and clinical cases (n=39) between 1995 and 2005 was determined by standard agar dilution and E-test methods. Antimicrobial drugs currently used in veterinary and human therapy were tested, and they included penicillin G, ampicillin, cephalothin, gentamicin, chloramphenicol, tetracycline, doxycycline, trimethoprim, erythromycin, and clindamycin. The sensitivity of strains was established using Clinical and Laboratory Standards Institute (formerly NCCLS) breakpoints and MIC50 (the MIC for 50% of the strains) to MIC90 values. In general, isolates were susceptible to the majority of the antimicrobials tested, including beta-lactamics and aminoglycosides, which are normally used in the treatment of listeriosis. Resistance to tetracycline and doxycycline was found in five strains isolated from fresh trout belonging to the same fish farm. Molecular analysis by restriction endonuclease analysis showed a similar profile, suggesting the persistence of a strain well adapted to the presence of tetracycline in the environment of a fish farm, which is frequently used in aquaculture in order to prevent infections of fish.     

Penicillin resistance in bovine Staphylococcus aureus: Genomic evaluation of the discrepancy between phenotypic and molecular test methods

Staphylococcus aureus is a major pathogen in humans and animals. In cattle, it is one of the most important agents of mastitis, causing serious costs in the dairy industry. Early diagnosis and adequate therapy are therefore 2 key factors to deal with the problems caused by this bacterium, and benzylpenicillin (penicillin) is usually the first choice to treat these infections. Unfortunately, penicillin resistance testing in bovine S. aureus strains shows discrepant results depending on the test used; consequently, the best method for assessing penicillin resistance is still unknown. The aim of this study was therefore to find a method that assesses penicillin resistance in S. aureus and to elucidate the mechanisms leading to the observed discrepancies. A total of 146 methicillin-sensitive S. aureus strains isolated from bovine mastitis were tested for penicillin resistance using a broth microdilution [minimum inhibitory concentration (MIC)] and 2 different disk diffusion protocols. Furthermore, the strains were analyzed for the presence of the bla operon genes (blaI, blaR1, blaZ) by PCR, and a subset of 45 strains was also subjected to whole genome sequencing (WGS). Discrepant results were obtained when penicillin resistance of bovine S. aureus was evaluated by disk diffusion, MIC, and PCR methods. The discrepancies, however, could be fully explained by WGS analysis. In fact, it turned out that penicillin resistance is highly dependent on the completeness of the bla operon promotor: when the bla operon was complete based on WGS analysis, all strains showed MIC ≥1 µg/mL, whereas when the bla operon was mutated (31-nucleotide deletion), they were penicillin sensitive except in those strains where an additional, bla operon-independent resistance mechanism was observed. Further, WGS analyses showed that penicillin resistance is truly assessed by the MIC assay. In contrast, caution is required when interpreting disk diffusion and PCR results.     

生乳中金黄色葡萄球菌产肠毒素特性及耐药性分析

了解生乳中金黄色葡萄球菌污染状况、产肠毒素特性及耐药性，为乳及乳制品的生产及制定HACCP以保证食品安全提供依据。采集省内20个奶牛场共209份生乳样品进行金黄色葡萄球菌的分离、产肠毒素（SEA—SEE）试验及耐药性试验。共检出金黄色葡萄球菌34株，检出率为16．27％，21株产肠毒素试验阳性，阳性率为61．76％，对青霉素、缸霉素、四环素、苯唑西林凝固酶阴性耐药性分别达88．24％、41．48％、35．29％、35．29％，并出现21种耐药谱。生乳中金黄色葡萄球菌的污染率及产肠毒素率较高，耐药性结果对人及畜的安全用药有指导性作用。控制生乳中金黄色葡萄球菌的污染对奶及奶制品食用安全有意义。