Udder pathogens and their resistance to antimicrobial agents in dairy cows in Estonia

Background

The goal of this study was to estimate the distribution of udder pathogens and their antibiotic resistance in Estonia during the years 2007-2009.

Methods

The bacteriological findings reported in this study originate from quarter milk samples collected from cows on Estonian dairy farms that had clinical or subclinical mastitis. The samples were submitted by local veterinarians to the Estonian Veterinary and Food Laboratory during 2007-2009. Milk samples were examined by conventional bacteriology. In vitro antimicrobial susceptibility testing was performed with the disc diffusion test. Logistic regression with a random herd effect to control for clustering was used for statistical analysis.

Results

During the study period, 3058 clinical mastitis samples from 190 farms and 5146 subclinical mastitis samples from 274 farms were investigated. Positive results were found in 57% of the samples (4680 out of 8204), and the proportion did not differ according to year (p > 0.05). The proportion of bacteriologically negative samples was 22.3% and that of mixed growth was 20.6%. Streptococcus uberis (Str. uberis) was the bacterium isolated most frequently (18.4%) from cases of clinical mastitis, followed by Escherichia coli (E. coli) (15.9%) and Streptococcus agalactiae (Str. agalactiae) (11.9%). The bacteria that caused subclinical mastitis were mainly Staphylococcus aureus (S. aureus) (20%) and coagulase-negative staphylococci (CNS) (15.4%). The probability of isolating S. aureus from milk samples was significantly higher on farms that had fewer than 30 cows, when compared with farms that had more than 100 cows (p < 0.005). A significantly higher risk of Str. agalactiae infection was found on farms with more than 600 cows (p = 0.034) compared with smaller farms. The proportion of S. aureus and CNS isolates that were resistant to penicillin was 61.4% and 38.5%, respectively. Among the E. coli isolates, ampicillin, streptomycin and tetracycline resistance were observed in 24.3%, 15.6% and 13.5%, respectively.

Conclusions

This study showed that the main pathogens associated with clinical mastitis were Str. uberis and E. coli. Subclinical mastitis was caused mainly by S. aureus and CNS. The number of S. aureus and Str. agalactiae isolates depended on herd size. Antimicrobial resistance was highly prevalent, especially penicillin resistance in S. aureus and CNS.     

Prevalence of bacterial pathogens and their anti-microbial resistance in Tilapia and their pond water in Trinidad

In Trinidad, Tilapia (Oreonchromis spp.) is one of the most important fresh water food fish and the number of farms has been increasing annually. A study was conducted in the local tilapia industry to determine the microbial quality of pond water, prevalence of bacterial pathogens and their anti-microbial resistance using the disk diffusion method. Seventy-five apparently healthy fish and 15 pond water samples from three of the four commercial tilapia fish farms in the country were processed. The 202 bacterial isolates recovered from fish slurry and 88 from water, belonged to 13 and 16 genera respectively. The predominant bacteria from fish slurry were Pseudomonas spp. (60.0%), Aeromonas spp. (44.0%), Plesiomonas (41.3%) and Chromobacterium (36.0%) (P < 0.05; chi(2)) compared with isolates from pond water where Bacillus spp. (80.0%), Staphylococcus spp., Alcaligenes spp. and Aeromonas spp. (60.0%) were most prevalent (P < 0.05; chi(2)). Using eight anti-microbial agents, to test bacteria from five genera (Aeromonas, Chromobacterium, Enterobacter, Plesiomonas and Pseudomonas), 168 (97.1%) of 173 bacterial isolates from fish slurry exhibited resistance to one or more anti-microbial agents compared with 47 (90.4%) of 52 from water (P > 0.05; chi(2)). Resistance was high to ampicillin, 90.2% (158 of 173), erythromycin, 66.5% (115 of 173) and oxytetracycline, 52.6%, (91 of 173) but relatively low to chloramphenicol, 9.8% (17 of 173) and sulphamethoxazole/trimethoprim, 6.4% (11 of 173) (P < 0.05; chi(2)). For pond water isolates, the frequency of resistance across bacterial genera ranged from 75% (nine of 12) for Chromobacter spp. to 100% found amongst Enterobacter spp. (six of six), Plesiomonas spp. (nine of nine) and Pseudomonas spp. (eight of eight) (P < 0.05; chi(2)). Resistance was generally high to ampicillin, 78.8% (41 of 52), erythromycin, 51.9% (27 of 52) and oxytetracycline, 34.5% (18 of 52) but low to sulphamethoxazole/trimethoprim, 7.7% (four of 52) and norfloxacin, 3.8% (two of 52) (P < 0.05; chi(2)). It was concluded that the rather high prevalence of bacterial pathogens in tilapia along with their high prevalence of resistance to anti-microbial agents might pose therapeutic problems as well as health risk to consumers. The microbial presence and their anti-microbial resistance in the tilapia industry are being reported for the first time in the country.     

Emerging antimicrobial resistance in commensal Escherichia coli with public health relevance

In 2009, 1462 Escherichia coli isolates were collected in a systematic resistance monitoring approach from primary production, slaughterhouses and at retail and evaluated on the basis of epidemiological cut-off values. Besides resistance to antimicrobial classes that have been extensively used for a long time (e.g. sulphonamides and tetracyclines), resistance to (fluoro)quinolones and third-generation cephalosporins was observed. While in the poultry production chain the majority (60%) of isolates from laying hens was susceptible to all antimicrobials tested, most isolates from broilers, chicken meat and turkey meat showed resistance to at least one (85-93%) but frequently even to several antimicrobial classes (73-84%). In the cattle and pig production chain, the share of isolates showing resistance to at least one antimicrobial was lowest (16%) in dairy cows, whereas resistance to at least one antimicrobial ranged between 43% and 73% in veal calves, veal and pork. Resistance rates to ciprofloxacin and nalidixic acid in isolates from broilers were 41.1% and 43.1%, respectively. Likewise, high resistance rates to (fluoro)quinolones were observed in isolates from chicken meat and turkey meat. In contrast, ciprofloxacin resistance was less frequent in E.?coli isolates from the cattle and pig production chain with highest rate in veal calves (13.3%). Highest resistance rates to cephalosporins were observed in broilers and chicken meat, with 5.9% and 6.2% of the isolates showing resistance. In dairy cattle and veal, no isolates with cephalosporin resistance were detected, whereas 3.3% of the isolates from veal calves showed resistance to ceftazidime. Resistance to (fluoro)quinolones and cephalosporins in E.?coli isolates is of special concern because they are critically important antimicrobials in human antimicrobial therapy. The emergence of this resistance warrants increased monitoring. Together with continuous monitoring of antimicrobial usage, management strategies should be regularly assessed and adapted.     

产IMP酶4种病原细菌的检测及其耐药性

采用16SrRNA和种特异基因的聚合酶链反应(polymerase chainreaction,PCR)鉴定菌株;用VITEK-AMS 60全自动微生物分析仪测定最小抑菌浓度(minimum inhibition concentration,MIC);PCR测序检测已知的超广谱β-内酰胺酶基因(extended spectrum beta-lactamases,ESBLs)和碳青霉烯酶基因;改良型Carba NP法检测碳青霉烯酶类型。对来自4家医院的6株高水平耐碳青霉烯类抗生素临床分离菌株的检测和耐药性研究。结果显示,6株分别为阴沟肠杆菌、铜绿假单胞菌、产酸克雷伯菌各1株,肺炎克雷伯菌3株,且均为blaIMP扩增阳性,并携带1个或多个blaTEM、blaCTX-M-1group、blaSHV、blaVIM、blaOXA-1基因。6株菌均产B型碳青霉烯酶,且对头孢菌素类和碳青霉烯类抗菌药物高水平耐药。结果表明,6株菌对头孢菌素类和碳青霉烯类抗菌药物多重耐药的主要决定因子是产B型碳青霉烯酶IMP,且和携带ESBLs基因也相关。     

Characterisation of ceftiofur resistance in swine bacterial pathogens

A PCR based method was developed for the identification of ceftiofur resistance genes (bla_CMY-2, bla_TEM-1, and ampC) in swine bacterial pathogens. Using this method, the ceftiofur resistant (n = 76) and susceptible (n = 45) strains of Bordetella bronchiseptica, Salmonella spp., Escherichia coli, and Pasteurella multocida were screened for the presence of these three genes. The resistant genes were detected in 70% (bla_TEM-1), 68% (bla_CMY-2) and 45% (ampC) of the resistant isolates and in 18% (bla_TEM-1), 27% (bla_CMY-2), and 36% (ampC) of the susceptible isolates. Results obtained in the present study showed widespread distribution of these three resistance genes in ceftiofur-resistant swine pathogens. It was also observed that more pathogens are acquiring these resistance genes.     

The distribution of pathogens and their antimicrobial susceptibility patterns among canine surgical wound infections in Sweden in relation to different risk factors

Background

Surgical site infection (SSI) is a common nosocomial infection in dogs and a growing concern in veterinary hospitals as an increase in multidrug-resistant pathogens is reported. Despite the need for rational and prudent antimicrobial use, few peer-reviewed and published veterinary studies have investigated the pathogenic growth including susceptibility patterns of the isolated pathogens in canine SSIs.The first objective of the present study was to estimate the distribution of bacterial pathogens in dogs with SSI and to investigate whether this was influenced by type of surgical procedure (clean, clean-contaminated, contaminated or dirty), duration of hospitalization, wound classification and depth of the infection, or antimicrobial treatment. The second objective was to assess susceptibility patterns to clinically relevant antimicrobials.During three years, four animal referral hospitals and three small animal clinics submitted bacterial swabs from canine SSIs for culture and susceptibility, together with a questionnaire completed by the attending clinician.

Results

Approximately two thirds of the in total 194 isolates were staphylococci. Staphylococcus pseudintermedius was the most prevalent finding (46%) followed by beta haemolytic Streptococcus spp. (24%). No associations between distribution of the isolated pathogens and classification of the surgical procedure, duration of hospitalization or depth of the SSI were shown, with the exception of Escherichia coli isolates being significantly more often found in deep wound infections than in superficial skin infections.Overall the possibilities of finding first generations antimicrobials to treat the SSIs included in the study were favorable, as the isolated pathogens were mostly without acquired antimicrobial resistance and multidrug resistance was uncommon. There were only three cases of methicillin-resistant S. pseudintermedius-infections (one percent of all isolates), one case of extended-spectrum beta-lactamase producing E. coli-infection, and no methicillin-resistant Staphylococcus aureus infections.

Conclusions

None of the investigated factors were shown to influence the distribution of bacterial pathogens. The majority of SSIs were caused by staphylococci, and S. pseudintermedius was the most prevalent pathogen. Based on the study results, use of first-line antimicrobials prior to receiving culture and susceptibility results is a rational empirical antimicrobial therapy for the studied dog population.     

Isolation and antimicrobial susceptibilities of bacterial pathogens from bovine pneumonia: 1994--2002.

Between 1994 and 2002, a total of 390 (46.3%) Mannheimia haemolytica, 292 (34.7%) Pasteurella multocida, and 160 (19.0%) Histophilus somni were isolated at the Oklahoma Animal Disease Diagnostic Laboratory from lungs from 6-18-month-old beef cattle with pneumonia. The ratio of M. haemolytica isolations to P. multocida isolations decreased from 3.1 in 1994 to 0.8 in 2000 while increasing to 1.5 in 2002. Mannheimia haemolytica isolations significantly (P < 0.05) decreased from 62.5% in 1994 to between 30.6% and 50.4% in 1998--2002. Pasteurella multocida isolations significantly (P < 0.05) increased from 20.0% in 1994 to between 28.6% and 47.4% in 1998--2002. Histophilus somni isolations were <19% except in 1998 (40.8%) and 1999 (23%). Antimicrobial susceptibilities for M. haemolytica significantly declined for erythromycin (P = 0.0001), florfenicol (P = 0.0004), spectinomycin (P = 0.0001), and tilmicosin (P = 0.03). For P. multocida, antimicrobial susceptibilities significantly declined for erythromycin (P = 0.0001), florfenicol (P = 0.004), spectinomycin (P = 0.03), sulfachloropyridizine (P = 0.028), tetracycline (P = 0.017), tilmicosin (P = 0.0001), and trimethoprim/sulfamethoxazole (P = 0.0003). Antimicrobial susceptibilities for H. somni were variable for spectinomycin and sulfachloropyridizine, whereas susceptibilities to other antibiotics remained consistently high.     

Fecal shedding and antimicrobial susceptibility of selected bacterial pathogens and a survey of intestinal parasites in free-living waterfowl

Free-living waterfowl residing in metropolitan parks in central Ohio were surveyed for the fecal shedding and antimicrobial susceptibility patterns of Campylobacter jejuni, Escherichia coli, Salmonella spp., and Pasteurella multocida. In addition, a survey for intestinal parasites was also conducted in these same waterfowl to determine parasite burdens in free-living waterfowl. Prevalences of 67%, 50%, and 0.2% of E. coli, C. jejuni, and Salmonella spp., respectively, were observed for all waterfowl species. Pasteurella multocida was not isolated from the sampled population. Salmonella java was isolated from one mallard duck. Statistically, there was a significantly higher E. coli isolation rate for mallard ducks than for Canada geese, but no difference was observed for C. jejuni isolation rates between waterfowl species. Antimicrobial susceptibility testing was conducted via the disk diffusion method and multidrug resistance was exhibited for penicillin G, lincomycin, vancomycin, erythromycin, and bacitracin. In addition, the prevalence of endoparasites in these sampled waterfowl ranged from 5% to 66%. Protozoan oocysts were most prevalent followed by nematode ova. No trematode or cestode ovum was recovered from this sampled population.     

2008～2009年西南地区规模化猪场病原菌耐药性流行情况调查

本实验室对西南地区47个规模化猪场的364份病料进行细菌分离,并采用形态、显微观察结合16S rDNA PCR扩增测序鉴定菌株;共检出细菌509株,细菌分离率达84.3%,混合感染率为37.4%;主要病原为大肠杆菌(30.4%)、葡萄球菌(21.6%)、肺炎克雷伯菌(8.7%)、变形杆菌(8.5%)、链球菌(5.9%)、肠球菌(11.4%)。小鼠试验结果显示平均致死率高达37.3%。191株致病菌株的药敏试验显示,β-内酰胺酶抑制剂药物(AMC,SAM,TIM)和头孢噻呋(EFT)耐药率较低(≤18.8%),其他药物的耐药率高(≥34.1),多重耐药严重,以5重、6重耐药为主。     

Trends of antimicrobial resistance in bacterial isolates from a small animal referral hospital

A longitudinal, retrospective investigation was made of antimicrobial resistance in bacterial isolates obtained from clinical cases in a small animal hospital between 1989 and 1997. Isolates of Escherichia coli and Staphylococcus species were used as Gram-negative and Gram-positive indicator organisms, respectively, and the annual prevalence of antimicrobial resistance was calculated for each organism to each of nine (for E coli) and 11 (for Staphylococcus species) appropriate antimicrobials, including enrofloxacin. Using a chi-square test for trend, statistically significant, rising trends were identified in the resistance of E coli to amoxycillin (P=0.04), clavulanate-amoxycillin (P<0.01) and streptomycin (P<0.01), and in the resistance of Staphylococcus species to erythromycin (P<0.01). There was an equivocal, rising trend for the resistance of Staphylococcus species to cephalexin. No significant trends were apparent for any of the other 15 organism/drug interactions. The annual prevalence of multiple drug resistance was calculated for E coli, Proteus species, Pseudomonas species, staphylococci and streptococci, but no statistically significant trends were identified.     

Monitoring of antimicrobial resistance among food animals: principles and limitations

Large amounts of antimicrobial agents are in the production of food animals used for therapy and prophylactics of bacterial infections and in feed to promote growth. The use of antimicrobial agents causes problems in the therapy of infections through the selection for resistance among bacteria pathogenic for animals or humans. Current knowledge regarding the occurrence of antimicrobial resistance in food animals, the quantitative impact of the use of different antimicrobial agents on selection for resistance and the most appropriate treatment regimes to limit the development of resistance is incomplete. Programmes monitoring the occurrence and development of resistance are essential to determine the most important areas for intervention and to monitor the effects of interventions. When designing a monitoring programme it is important to decide on the purpose of the programme. Thus, there are major differences between programmes designed to detect changes in a national population, individual herds or groups of animals. In addition, programmes have to be designed differently according to whether the aim is to determine changes in resistance for all antimicrobial agents or only the antimicrobial agents considered most important in relation to treatment of humans. In 1995 a continuous surveillance for antimicrobial resistance among bacteria isolated from food animals was established in Denmark. Three categories of bacteria, indicator bacteria, zoonotic bacteria and animal pathogens are continuously isolated from broilers, cattle and pigs and tested for susceptibility to antimicrobial agents used for therapy and growth promotion by disc diffusion or minimal inhibitory concentration determinations. This programme will only detect changes on a national level. However, isolating the bacteria and testing for several antimicrobial agents will enable us to determine the effect of linkage of resistance. Since 1995 major differences in the consumption pattern of different antimicrobial agents have occurred in Denmark. The Danish monitoring programme has enabled us to determine the effect of these changes on the occurrence of resistance. The Danish monitoring is, however, not suited to determine changes on a herd level or to detect emergence of new types of resistance only occurring at a low level.     

Campylobacter spp in human,chickens, pigs and their antimicrobial resistance

Campylobacter spp. have been identified as etiologic agents in outbreaks and sporadic cases of gastroenteritis in developed countries. In developing countries, most reported Campylobacter infections are in children. Previously reported prevalences of Campylobacter spp. in children in Southeast Asia range from 2.9% to 15%. The frequency and pattern of occurrence of Campylobacter spp. differ between developed and developing countries, especially in the number of cases reported in adults and the presence of any seasonal patterns in occurrence. Although the severity of Campylobacter infection in adults was different between developed and developing countries, the clinical symptoms of infection in adults resulting from infection in developing countries was similar to those in developed countries. Many different animal species maintain Campylobacter spp. with no clinical signs. There do not appear to be significantly different colonization rates of Campylobacter in food animals between developed and developing countries. The role of C. jejuni as a primary pathogen in farm animals is uncertain. C. jejuni can be found in feces of diarrheic and healthy calves and piglets. Campylobacter with resistance to antimicrobial agents have been reported in both developed and developing countries, and the situation seems to deteriorate more rapidly in developing countries, where there is widespread and uncontrolled use of antibiotics resistance was observed at high levels in food animals in both developed and developing countries. Studies suggested an association between antimicrobial use in food animals and the development of resistance in human isolates in developed countries.     

The effect of rehabilitation of northern elephant seals (Mirounga angustirostris) on antimicrobial resistance of commensal Escherichia coli

The aim of this study was to determine if antimicrobial drug use increases resistance of commensal gastrointensinal Escherichia coli of wild northern elephant seals (Mirounga angustirostris) treated in rehabilitation, and, if so, identify the risk factors involved. Minimum inhibitory concentration (MIC) levels of twelve antimicrobial drugs were determined for 289 E. coli isolates from 99 seals sampled at admission and 277 isolates obtained at release from rehabilitation using broth microdilution. Prevalence of E. coli antimicrobial resistance, MIC(50), MIC(90), and clustering of MIC values were determined for seals and the data were analyzed using Fisher's exact test, ordinal logistic regression and negative binomial regression. At release from rehabilitation 77.8% of the seals had antimicrobial resistant E. coli compared to 38.4% of the seals at admission. The MIC(90) for amoxicillin-clavulanic acid, chloramphenicol, enrofloxacin, ticarcillin-clavulanic acid, and trimethoprim-saulfamethoxazole were at levels considered to be sensitive at admission but they increased to levels of resistance at release. E. coli were grouped into four clusters by their MIC values, with increasing levels of resistance going from Cluster 1 to 4. A primary risk factor associated with the probability of a seal having E. coli in Clusters 3 and 4 was time in rehabilitation, regardless of whether the animal received treatment with antimicrobial drugs, suggesting nosocomial infection. The results of this study provide evidence that increased levels of hygiene and appropriate use of antimicrobial therapy might be important in the rehabilitation of wild animals to prevent rise in the prevalence of antimicrobial resistant bacteria.     

Usage of antimicrobials and occurrence of antimicrobial resistance among bacteria from mink

The usage of antimicrobials for treatment of mink and the occurrence of antimicrobial resistance among the most important bacterial pathogens in mink was investigated. The aim of the study was to provide data, which may serve as a basis for the formulation of recommendations for prudent use of antimicrobials for mink. A total of 164 haemolytic staphylococci, 49 haemolytic streptococci, 39 Pseudomonas aeruginosa, 13 Pasteurella multocida, and 1093 Escherichia coli isolates from Danish mink were included in the study. A high frequency of resistance among S. intermedius was found for tetracyclines (54.7%), followed by penicillin (21.7%), lincosamides (20.4%), macrolides (19.1%), and spectinomycin (18.5%). Very low frequencies of resistance were recorded for other antimicrobials. The highest frequency among the E. coli isolates was recorded for ampicillin, streptomycin, sulphonamides, and tetracyclines, whereas resistance to other antimicrobials was rare. All P. aeruginosa were sensitive to gentamicin and colistin and sensitive or intermediate to enrofloxacin, whereas most isolates were resistant to all other antimicrobials. All P. multocida and haemolytic streptococci were sensitive to penicillin. There was a steady increase in the use of antimicrobials during the period 2001-2006, the majority of the prescribed amount being extended spectrum penicillins followed by aminoglycosides, sulphonamides with trimethoprim, and macrolides.     

Comparison of antimicrobial resistance pattern of selected respiratory tract pathogens isolated from different animal species

The antibiotic resistance pattern of respiratory tract pathogens isolated of different animal species suffering from respiratory tract diseases has been investigated by antibiograms performed by agar diffusion test. The results show that the resistance situation in Switzerland is favourable compared with studies from other countries. However, high resistance rates were found in certain species: 61% of Streptococcus spp. were resistant to erythromycin and 44% to tetracycline, 59% of Bordetella bronchiseptica were resistant to ampicillin and 50% of Mannheimia (Pasteurella) haemolytica were multiresistant to tetracycline, ampicillin and streptomycine. The gram negative isolates were widely resistant to streptomycine.     

Retrospective study of bacterial isolates and their antimicrobial susceptibilities in equine uteri during fertility problems

Bacterial pathogens are a potential cause when a mare fails to conceive to a fertile stallion on a well-managed breeding farm on one or more cycles in the same season. Furthermore, emerging bacterial resistance to commonly used (topical) antibiotics has been demonstrated. In this study, a total of 586 uterine swabs from mares with fertility problems were evaluated and the bacterial isolates were identified and measured for resistance to 10 antibiotics most commonly used during bacterial equine infection. Forty-nine percent of the examined mares were positive at bacteriological investigations. Amongst 347 successful isolations, 31.7% were Streptococcus group C and 18.4% Escherichia (E.) coli, both considered frequently associated with fertility problems. Determination of the antibiotic susceptibility pattern of Streptococcus group C (110 organisms) revealed that only the amoxicillin/clavulanic acid was highly active with 82.7% of the isolates being inhibited. For E. coli, a major number of drugs displayed a high potency.     

Identification through MALDI-TOF mass spectrometry and antimicrobial susceptibility profiling of bacterial pathogens isolated from sow urinary tract infection

Background: Urinary tract infection (UTI) is a common disease in sows due to intensification of pig production. Despite direct economic losses, UTI prevalence and respective microbial identification are still poorly studied.

Objective: The aims of this study were to identify the causative agents of UTI in sows through MALDI-TOF MS and to characterize their antimicrobial resistance profiles.

Materials and Methods: Urine samples from 300 sows of three herds from São Paulo State (Brazil) were screened for UTI; suggestive samples were submitted to bacterial isolation. Species identification was performed by MALDI-TOF MS and susceptibility profiles were determined using disc diffusion method.

Results: 128 samples suggestive of UTI were analyzed; 48% of the animals presented UTI caused by a single pathogen, while the remaining 52% presented mixed infection. Escherichia coli stood out with the highest frequency among both single and mixed infections. The Gram-positive were exclusively associated with 27% of single infections. The mixed infections were further classified into 49 profiles. The high frequency of multiresistant profiles stood out for most of the studied isolates.

Conclusions: MALDI-TOF MS enabled the identification of rare pathogens related to UTI which may represent higher risk for porcine health, especially considering high frequency of multiresistant profiles.     

Pneumonia in calves: characterization of the bacterial spectrum and the resistance patterns to antimicrobial drugs

The population under study included young calves with pneumonia (group A, n = 13) and their controls (group B, n = 9), as well as older calves from which the lungs with (group C, n = 90) or without (group D, n = 10) lesions were collected after slaughter. Arcanobacterium pyogenes was the organism most commonly isolated from calves in group A (46%), followed by Haemophilus somnus (23%), Mannheimia haemolytica (15%), Streptococcus suis and Pasteurella multocida (7.7% each). Only S. suis (22%) and P. multocida (11%) were found in group B. P. multocida was isolated from 32% group C calves, H. somnus from 11%, A. pyogenes from 7.8%, M. haemolytica from 2.2% and S. suis from 1.1%. No specific pathogens were isolated in group D. Prevalence of Mycoplasma bovis infection was 69% in group A and 37% in group C. Ninety-eight strains were tested for resistAnce to antibiotics. Resistance to penicillin and ampicillin was present only in M. haemolytica (46%). High percentages of resistant strains were observed for streptomycin (48-100%), tetracycline (15-43%), sulfonamides alone (14-100%) or in combination with trimethoprim (0-100%). Therapeutic approaches to bacterial calf pneumonia in the area under study should be modified according to the isolated bacterial population, the observed antimicrobial resistances and the growing importance of Mycoplasma bovis.