Serotype and apx genotype profiles of Actinobacillus pleuropneumoniae field isolates in Korea

Brazilian Clinical Strains of Actinobacillus pleuropneumoniae and Pasteurella multocida: Capsular Diversity, Antimicrobial Susceptibility (In Vitro) and Proof of Concept for Prevention of Natural Colonization by Multi-Doses Protocol of Tildipirosin

One hundred Actinobacillus pleuropneumoniae (App) and sixty Pasteurella multocida subsp. multocida serogroup A (PmA) isolates were recovered from porcine pneumonic lungs collected from eight central or southern states of Brazil between 2014 and 2018 (App) or between 2017 and 2021 (PmA). A. pleuropneumoniae clinical isolates were typed by multiplex PCR and the most prevalent serovars were 8, 7 and 5 (43, 25% and 18%, respectively). In addition, three virulence genes were assessed in P. multocida isolates, all being positive to capA (PmA) and kmt1 genes, all negative to capD and toxA, and most of them (85%) negative to pfhA gene. The susceptibility of both pathogens to tildipirosin was investigated using a broth microdilution assay. The percentage of isolates susceptible to tildipirosin was 95% for App and 73.3% for PmA. The MIC50 values were 0.25 and 1 μg/mL and the MIC90 values were 4 and >64 μg/mL for App and PmA, respectively. Finally, a multiple-dose protocol of tildipirosin was tested in suckling piglets on a farm endemic for both pathogens. Tildipirosin was able to prevent the natural colonization of the tonsils by App and PmA and significantly (p < 0.0001) reduced the burden of Glaesserella parasuis in this tissue. In summary, our results demonstrate that: (i) tildipirosin can be included in the list of antibiotics to control outbreaks of lung disease caused by App regardless of the capsular type, and (ii) in the case of clinical strains of App and PmA that are sensitive to tildipirosin based on susceptibility testing, the use of this antibiotic in eradication programs for A. pleuropneumoniae and P. multocida can be strongly recommended.

Comparative Efficacy in Challenge Dose Models of a Toxin Expressing Whole-Cell Vaccine against Eight Serovars of Actinobacillus pleuropneumoniae in Pigs

Actinobacillus pleuropneumoniae is a major economically significant bacterial respiratory pig pathogen, and whole cell vaccines are used to prevent disease. However, there is little data available on multi-serovar whole cell vaccine protection. Therefore, we determined the protective efficacies of a whole-cell A. pleuropneumoniae serovar 1 and 2 vaccine comprising ApxI-III toxins (C-vaccine, Coglapix^®, Ceva, France) against serovars 1, 2, 4, 5, 6, 7, 9/11, and 13. The infection doses used induced disease representative of endemic field conditions, and standard protocols were used for all studies. Protection against homologous serovars 1 and 2 significantly reduced lung lesion scores (LLS) compared to positive controls: p = 0.00007 and p = 0.00124, respectively. The protection against heterologous serovars 4, 5, 6, 7, 9/11, and 13 also significantly reduced LLS: range p = 2.9 × 10^-10 to p = 0.00953. As adjudged by the estimated random effect, reproducibility between studies was high. A highly significant serovar-independent reduction of pathological lung lesions by the C-vaccine was found for all the serovars tested (1, 2, 4, 5, 6, 7, 9/11, and 13). We conclude that the C-vaccine gives high serovar-independent protection against disease and is suitable for this use in the field.

Actinobacillus pleuropneumoniae: The molecular determinants of virulence and pathogenesis

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is responsible for high economic losses in swine herds across the globe. Pleuropneumonia is characterized by severe respiratory distress and high mortality. The knowledge about the interaction between bacterium and host within the porcine respiratory tract has improved significantly in recent years. A. pleuropneumoniae expresses multiple virulence factors, which are required for colonization, immune clearance, and tissue damage. Although vaccines are used to protect swine herds against A. pleuropneumoniae infection, they do not offer complete coverage, and often only protect against the serovar, or serovars, used to prepare the vaccine. This review will summarize the role of individual A. pleuropneumoniae virulence factors that are required during key stages of pathogenesis and disease progression, and highlight progress made toward developing effective and broadly protective vaccines against an organism of great importance to global agriculture and food production.

Sero- and apx-typing of German Actinobacillus pleuropneumoniae field isolates from 2010 to 2019 reveals a predominance of serovar 2 with regular apx-profile

Serotyping is the most common method to characterize field isolates of Actinobacillus (A.) pleuropneumoniae, the etiological agent of porcine pleuropneumonia. Based on serology, many farms seem to be infected and antibodies against a wide variety of serovars are detectable, but, so far it is unknown to what degree respective serovars contribute to outbreaks of clinical manifest disease. In this study, 213 German A. pleuropneumoniae field isolates retrieved for diagnostic purposes from outbreaks of porcine pleuropneumonia between 2010 and 2019 were genetically serotyped and analyzed regarding their apx-toxin gene profile using molecular methods. Serotyping revealed a prominent role of serovar 2 in clinical cases (64% of all isolates) and an increase in the detection of this serovar since 2010 in German isolates. Serovar 9/11 followed as the second most frequent serovar with about 15% of the isolates. Furthermore, very recently described serovars 16 (n = 2) and 18 (n = 8) were detected. Most isolates (93.4%) showed apx-profiles typical for the respective serovar. However, this does not hold true for isolates of serovar 18, as 75% (n = 6) of all isolates of this serovar deviated uniformly from the “typical” apx-gene profile of the reference strain 7311555. Notably, isolates from systemic lesions such as joints or meninges did not harbor the complete apxICABD operon which is considered typical for highly virulent strains. Furthermore, the extremely low occurrence (n = 1) of NAD independent (biovar II) isolates in German A. pleuropneumoniae was evident in our collection of clinical isolates.

Serotyping reanalysis of unserotypable Actinobacillus pleuropneumoniae isolates by agar gel diffusion test

We observed increasing unserotypable (UT) Actinobacillus pleuropneumoniae isolates using agar gel diffusion (AGD) test. To reanalyze their serovar, we performed rapid slide agglutination (RSA) test and multiplex PCR for 47 UT isolates. Of these, 25 were serovar 1 (UT-serovar 1), 20 were serovar 2 (UT-serovar 2) and 2 were serovar 15 (UT-serovar 15). We examined serotyping antigen extraction temperature to determine heat influence. UT-serovar 1 and 15 were influenced by heat, because their precipitation lines were observed in the case of low antigen extraction temperature. To investigate the relationship between antigenicity and genotype, we performed pulsed-field gel electrophoresis (PFGE) analysis using UT-serovar 2 and 15. The predominant PFGE pattern of UT-serovar 2 was identical to that of serovar 2.

Nasal immunization with M cell-targeting ligand-conjugated ApxIIA toxin fragment induces protective immunity against Actinobacillus pleuropneumoniae infection in a murine model

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia and severe economic loss in the swine industry has been caused by the infection. Therefore, the development of an effective vaccine against the bacteria is necessary. ApxII toxin, among several virulence factors expressed by the bacteria, is considered to be a promising vaccine candidate because ApxII toxin not only accompanies cytotoxic and hemolytic activities, but is also expressed in all 15 serotypes of bacteria except serotypes 10 and 14. In this study, we identified the peptide ligand capable of targeting the ligand-conjugated ApxIIA #5 fragment antigen to nasopharynx-associated lymphoid tissue. It was found that nasal immunization with ligand-conjugated ApxIIA #5 induced efficient mucosal and systemic immune responses measured at the levels of antigen-specific antibodies, cytokine-secreting cells after antigen exposure, and antigen-specific lymphocyte proliferation. More importantly, the nasal immunization induced protective immunity against nasal challenge infection of the bacteria, which was confirmed by histopathological studies and bacterial clearance after challenge infection. Collectively, we confirmed that the ligand capable of targeting the ligand-conjugated antigen to nasopharynx-associated lymphoid tissue can be used as an effective nasal vaccine adjuvant to induce protective immunity against A. pleuropneumoniae infection.

Functional pentameric formation via coexpression of the Escherichia coli heat-labile enterotoxin B subunit and its fusion protein subunit with a neutralizing epitope of ApxIIA exotoxin improves the mucosal immunogenicity and protection against challenge by Actinobacillus pleuropneumoniae

A coexpression strategy in Saccharomyces cerevisiae using episomal and integrative vectors for the Escherichia coli heat-labile enterotoxin B subunit (LTB) and a fusion protein of an ApxIIA toxin epitope produced by Actinobacillus pleuropneumoniae coupled to LTB, respectively, was adapted for the hetero-oligomerization of LTB and the LTB fusion construct. Enzyme-linked immunosorbent assay (ELISA) with GM1 ganglioside indicated that the LTB fusion construct, along with LTB, was oligomerized to make the functional heteropentameric form, which can bind to receptors on the mucosal epithelium. The antigen-specific antibody titer of mice orally administered antigen was increased when using recombinant yeast coexpressing the pentameric form instead of recombinant yeast expressing either the LTB fusion form or antigen alone. Better protection against challenge infection with A. pleuropneumoniae was also observed for coexpression in recombinant yeast compared with others. The present study clearly indicated that the coexpression strategy enabled the LTB fusion construct to participate in the pentameric formation, resulting in an improved induction of systemic and mucosal immune responses.

An immunosorbent assay based on the recombinant ApxIa, ApxIIa, and ApxIIIa toxins of Actinobacillus pleuropneumoniae and its application to field sera

Actinobacillus pleuropneumoniae is the etiologic agent of porcine pleuropneumonia, a highly contagious pulmonary disease in pigs with major economic losses for pig producers worldwide. Whereas A. pleuropneumoniae isolates are divided into 15 serotypes, the isolates secrete 4 types of exotoxins (ApxI, ApxII, ApxIII, and ApxIV), which are known as major virulence factors. In the current study, the ApxIA, ApxIIA, and ApxIIIA genes were amplified and their recombinant proteins expressed in Escherichia coli M15 cells. The antigenicity of each recombinant protein was demonstrated by Western blot and enzyme-linked immunosorbent assay (ELISA) using sera from pigs vaccinated with a subunit vaccine. When ELISAs using the recombinant antigens were optimized and then applied to sera from 320 randomized pigs in Korea, an observed increase in seroprevalence was found among sows in comparison with weaned piglets and growing pigs, indicating an age-dependent seroprevalence. The results obtained in the study suggest that the developed ELISAs may be useful for A. pleuropneumoniae vaccination strategy as a screening tool for pig herds as well as for detection of specific antibodies to Apx exotoxins.

Genetic diversity of Actinobacillus pleuropneumoniae assessed by amplified fragment length polymorphism analysis

Amplified fragment length polymorphism (AFLP) was evaluated as a method for genotypic characterization and subtyping within the bacterial species Actinobacillus pleuropneumoniae. A total of 155 isolates of A. pleuropneumoniae, representing the serotypic variation described to occur within this species, were analyzed. In order to elucidate the species boundaries, six strains of the phylogenetically closely related species Actinobacillus lignieresii were also included. Furthermore, the ability of AFLP to subtype was studied using 42 isolates of serovar 2 and the performance compared to that obtained by pulsed-field gel electrophoresis (PFGE). AFLP analysis provided a clear separation of A. lignieresii and A. pleuropneumoniae and divided the isolates of A. pleuropneumoniae into 20 clusters. Most of the serovars of A. pleuropneumoniae were represented by single and quite homogeneous clusters. The exceptions were serovars 10, K2:O7, and K1:O7, which were represented by two clusters each. In the cases where the serovars were represented by more than one cluster, the existence of these clusters was supported by additional phenotypic or genotypic properties. Furthermore, AFLP typing was able to allocate serologically nontypeable isolates to appropriate genetic groups within the species. Further investigations are needed to determine whether some of the clusters revealed through AFLP analysis represent additional serovars. When evaluated as a method for subtyping within serovar 2 of A. pleuropneumoniae, AFLP was found to achieve a degree of separation among isolates superior to that obtained by PFGE. However, a higher degree of separation between serovar 2 isolates was obtained by a combination of the two methods.

Development and use of a multiplex polymerase chain reaction assay based on Apx toxin genes for genotyping of Actinobacillus pleuropneumoniae isolates

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is the etiological agent of a porcine pleuropneumonia that threatens the global swine industry. The major pathogenic toxins of A. pleuropneumoniae include ApxI, ApxII, ApxIII, and ApxIV, which are serotype or serovar specific. Several techniques have been developed for the identification and typing of A. pleuropneumoniae. Serological assays are used to identify and serotype A. pleuropneumoniae, but factors such as cross-reactivity limit their specificity. Labor, time, and the requirement for specific antibodies are also drawbacks of these assays. Multistep polymerase chain reaction (PCR) techniques based on apx genes have been reported for the identification and typing of A. pleuropneumoniae. This study developed multiplex PCR for the identification and genotyping of A. pleuropneumoniae based on apx genes. This multiplex PCR technique was successful in differentiating 11 of 15 reference serotypes. Five different primer sets were used to amplify the 4 apx genes from each serotype in a single-step reaction. The multiplex PCR reported in this study was further used in genotyping 51 field isolates of A. pleuropneumoniae from different regions of Korea. The concomitant amplification of all 4 apx genes makes multiplex PCR more specific and convenient for the diagnosis and genotyping of A. pleuropneumoniae.

Cloning and expression of genes encoding transferrin-binding protein A and B from Actinobacillus pleuropneumoniae serotype 5

Actinobacillus pleuropneumoniae is an important primary pathogen in pigs, which causes a highly contagious pleuropneumonia. As an adaptation to the iron-restricted environment of the host, A. pleuropneumoniae possesses iron acquisition pathways mediated by surface receptors that specifically bind transferrin from the host. The receptor is composed of two receptor proteins, transferrin-binding protein A and B (TbpA and B), which are both capable of binding to transferrin. An impairment of iron uptake mechanisms is likely to reduce virulence. For this reason, these two proteins can be useful as a candidate target for A. pleuropneumoniae vaccination. To do this, genes encoding the TbpA and B from a serotype 5 isolate of A. pleuropneumoniae were amplified from genomic DNA template by PCR and cloned into a pRSET prokaryotic expression vector, generating the pRSET-A.pp-TbpA and B. Escherichia coli BL21(DE3)pLysS competent cells were transformed with each construct followed by the induction of protein expression by the addition of IPTG. Bands corresponding to the predicted sizes (110 and 60 kDa) were seen on the SDS-PAGE. Polyclonal antibodies raised against recombinant TbpA and B from mice were reacted with bacterial proteins. This result indicates that the recombinant proteins can induce immunological responses and might be useful as candidate targets for A. pleuropneumoniae vaccination.

Detection of nuclear factor-kappaB and inducible nitric oxide synthase in the lungs of pigs naturally infected with Actinobacillus pleuropneumoniae

Activated nuclear factor-kappaB (NF-kappaB) and inducible nitric oxide synthase (iNOS) were detected immunohistochemically in pleuropneumonic lungs from 20 pigs naturally infected with Actinobacillus pleuropneumoniae. NF-kappaB was detected mainly in nuclei of inflammatory cells, confirming its activation. Intense immunolabelling for NF-kappaB and iNOS was seen within the lung lesions, but labelling was minimal in unaffected portions of the lung of infected pigs and in normal lung from uninfected (control) pigs. Examination of serial sections from the 20 infected lung samples demonstrated a close association between NF-kappaB and iNOS. This suggests that NF-kappaB plays a key role in triggering the activation of iNOS in porcine pleuropneumonia.

In vitro effects of Actinobacillus pleuropneumoniae on inducible nitric oxide synthase and cyclooxygenase-2 in porcine alveolar macrophages

OBJECTIVE

To determine the amount of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) activity in alveolar macrophages in response to Actinobacillus pleuropneumoniae (APP) by determining nitric oxide (NO) and prostaglandin E2 (PGE2) concentrations.

SAMPLE POPULATION

Freshly isolated porcine alveolar macrophages.

PROCEDURE

Alveolar macrophages were incubated for 48 hours with APP (1 X 10(4) colony-forming units/mL), interleukin-1beta, (IL-1beta; 5 U/mL), tumor necrosis factor-alpha (TNFalpha; 500 U/mL), interferon-gamma (IFN-gamma, 100 U/mL), or lipopolysaccharide (LPS; 10 microg/mL). In a second experiment, alveolar macrophages were incubated with fresh medium (negative control), APP alone, or APP with 1 of the following: IL-1beta, TNF-alpha, or IFN-gamma. In a third experiment, alveolar macrophages were incubated with fresh medium (negative control), LPS (positive control), APP alone, or APP with 1 of the following: an iNOS inhibitor (3.3 microM), a COX-2 inhibitor (10 microM); or both the iNOS and COX-2 inhibitors. Supernatant was obtained at 0, 3, 6, 9, 12, 24, and 48 hours after treatment for determination of NO and PGE2 production.

RESULTS

The addition of APP to alveolar macrophages resulted in significant increases in NO and PGE2 production. The addition of APP and IFN-gamma synergistically induced NO production. Inhibition of iNOS and COX-2 decreased NO and PGE2 production, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro activation of alveolar macrophages by APP results in increased production of NO and PGE2. Nitric oxide and PGE2 production appears to be largely dependent on iNOS and COX-2 activity. Pharmacologic modulation of iNOS and COX-2 activity may represent a therapeutic target for pigs with pleuropneumonia.

PCR detection of Actinobacillus pleuropneumoniaeapxIV gene in formalin-fixed, paraffin-embedded lung tissues and comparison with in situ hybridization

AIMS

Formalin-fixed, paraffin-embedded lung tissues from pigs experimentally infected with 12 Actinobacillus pleuropneumoniae serotypes were used to develop nested PCR for the detection of apxIV gene.

METHODS AND RESULTS

The PCR results from formalin-fixed, paraffin-embedded tissues were compared with in situ hybridization. The apxIV gene was detected in formalin-fixed, paraffin-embedded lung tissues from all 39 pigs experimentally infected with 12 A. pleuropneumoniae serotypes by nested PCR. In situ hybridization produced a distinct positive signal in all 39 pigs experimentally infected with 12 A. pleuropneumoniae serotypes. Agreement rates between nested PCR and in situ hybridization were 100% for the detection of apxIV gene in formalin-fixed paraffin-embedded lung tissues. Acceptable PCR signals were detected from lung tissues fixed for periods up to 180 days.

CONCLUSIONS

The apxIV gene is species-specific rather than serotype-specific and is therefore an important diagnostic marker. The nested PCR assay would be a useful method for the detection of apxIV gene to diagnose A. pleuropneumoniae infection when formalin-fixed tissues are submitted.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study confirmed the possibility of using formalin-fixed, paraffin-embedded tissues for the diagnosis of A. pleuropneumoniae infection in pigs.

Differentiation of twelve Actinobacillus pleuropneumoniae serotypes by outer membrane lipoprotein gene-based restriction fragment length polymorphism

Twelve Actinobacillus pleuropneumoniae serotypes were differentiated by restriction fragment length polymorphism (RFLP) of polymerase chain reaction (PCR)-amplified fragments from the outer membrane lipoprotein (omlA) gene. All 12 reference serotypes and 80 field isolates produced the expected 950-base pair (bp) fragment of the omlA gene by PCR. Combining the RFLP patterns obtained with SfaNI, Bst71I, AluI, NciI, nine distinct patterns were observed in the 12 serotype reference strains. The PCR-based RFLP analysis of omlA genes allows differentiation among the 12 serotypes, with the exception of group 1 (serotypes 1, 9 and 11), and group 2 (serotypes 2 and 8). When the PCR products from the 70 field isolates were subjected to RFLP analysis, 68 showed the same RFLP patterns as their respective serotype reference strain. Two isolates that could not be typed had the same RFLP patterns as those of serotype 5. These results suggest that PCR-based RFLP analysis of the omlA genes may be of value in differentiating among 12 A. pleuropneumoniae serotypes.

Immunohistochemical detection of cyclooxygenase-2 in lungs of pigs naturally infected with Actinobacillus pleuropneumoniae

Cyclooxygenase-2 (COX-2) protein was detected immunohistochemically in formalin-fixed, paraffin wax-embedded lung tissues from 15 pigs with naturally occurring pleuropneumonia caused by Actinobacillus pleuropneumoniae. Positive cells typically exhibited a red reaction product without background staining. Alveolar macrophages, neutrophils, and bronchial and bronchiolar epithelial cells had positive immunohistochemical signals. Immunoreactivity of COX-2 protein was intense in the clustered leucocytes with streaming nuclear chromatin that are a characteristic histological feature of porcine pleuropneumonia. COX-2 protein was always associated with macrophages and neutrophils in pleuropneumonic lung lesions but was minimal in non-lesional lung of A. pleuropneumoniae -infected pigs and in normal lung from control pigs. The results suggest that COX-2 plays a role in pathophysiological processes during A. pleuropneumoniae infection.

Immunohistochemical detection and distribution of inducible nitric oxide synthase in pigs naturally infected with Actinobacillus pleuropneumoniae

Inducible nitric oxide synthase (iNOS) protein was detected immunohistochemically in formalin-fixed, paraffin wax-embedded lung tissues from 10 natural cases of porcine pleuropneumonia. Positive cells typically exhibited a red reaction product without background staining. Labelling of iNOS protein was intense in "oat cells", the clustered leucocytes with streaming nuclear chromatin that are a characteristic histological feature of porcine pleuropneumonia. Macrophages and neutrophils within alveolar spaces but not within blood vessels consistently showed iNOS labelling, but such labelling was minimal in non-lesional lung of Actinobacillus pleuropneumoniae -infected pigs and in normal lung from control pigs. The results suggest that iNOS plays a role in pathophysiological processes during A. pleuropneumoniae infection.

Expression of the apxIV gene in pigs naturally infected with Actinobacillus pleuropneumoniae

The apxIV gene was detected, by in-situ hybridization with a non-radioactive digoxigenin-labelled probe, in formalin-fixed, paraffin wax-embedded samples of lung tissue from 10 pigs naturally infected with Actinobacillus pleuropneumoniae. A 442 base pair DNA probe of the apxIV gene from A. pleuropneumoniae serotype 2 was generated by the polymerase chain reaction. All 10 pigs infected with A. pleuropneumoniae serotypes 2, 5, 6, or an untypable strain showed a distinct, positive signal in the degenerate alveolar leucocytes in alveolar spaces, and in the dense zone of degenerated cells in granulation tissue surrounding the necrotic areas. Thus, the study demonstrated the presence of the apxIV gene in pleuropneumonic lesions caused by A. pleuropneumoniae.

Detection and localization of ApxI, -II and -III genes of Actinobacillus pleuropneumoniae in natural porcine pleuropneumonia in natural porcine pleuropneumonia by in situ hybridization

In situ hybridization techniques that employed a nonradioactive digoxigenin-labeled probe were used to detect and localize ApxI, II and III genes in tissue sections of pneumonic lung naturally infected with Actinobacillus pleuropneumoniae. In pigs infected with either serotype 2 or 6, a hybridization signal for apxIICA, apxIIICA, apxIBD, and apxIIIBD was detected, and in pigs infected with serotype 5, a hybridization signal for apxICA, apxIICA, and apxIBD was detected in the pneumonic lesions. A hybridization signal for apxIICA and apxIBD was detected in pigs infected with serotype 7. A strong hybridization signal for apx genes was seen in streaming degenerate alveolar leukocytes bordering zones of coagulative necrosis. Simultaneous detection of hybridization signals for the apxCA and apxBD genes provided scientific evidence that the expression of the apx genes could be potential indicators of the production of corresponding Apx toxins. This study demonstrates the expression of ApxI, II, and III genes in pneumonic lesions caused by A. pleuropneumoniae.

Genotypic prevalence of apx1V in Actinobacillus pleuropneumoniae field isolates

A total of 90 Actinobacillus pleuropneumoniae field strains from pigs were serotyped by slide agglutination and analyzed for the presence of the apxIV gene by polymerase chain reaction. Of the 90 isolates serotyped, serotypes 2 (47 isolates) and 5 (25 isolates) were the most common, followed by serotype 6 (10 isolates). Three isolates belonged to serotype 7, and 5 isolates could not be typed. All 90 A. pleuropneumoniae field isolates tested carried the apxIV gene. This gene is species specific rather than serotype specific. Therefore, the ApxIV toxin has potential value for use both in vaccines and in diagnostic tests.

Antimicrobial susceptibility of Actinobacillus pleuropneumoniae isolated from pigs in Korea using new standardized procedures

The in vitro susceptibilities of 76 isolates of Actinobacillus pleuropneumoniae collected from pigs with pleuropneumonia were tested with 12 commonly used antimicrobial drugs by an agar dilution minimal inhibitory concentration procedure according to National Committee for Clinical Laboratory Standards (NCCLS) guidelines. Field isolates had low MICs for ceftiofur, danofloxacin and penicillin. No correlation of antimicrobial resistance was related to serotype.