1. Introduction
Mannheimia haemolytica is an important ruminant pathogen contributing to bovine respiratory disease (BRD) in cattle and pneumonic pasteurellosis in sheep and leading to a large proportion of the overall antimicrobial usage in both cattle and sheep [
1].
M. haemolytica is a commensal in the nasopharynx that can lead to clinical disease usually when animals are exposed to predisposing factors, which include stress induced by change in the environment or by other bacterial and viral infections [
2]. In sheep, pneumonic pasteurellosis can course as acute disease, but subclinical or chronic cases are frequent, and brochopneumonic lesions derived from these presentations are common findings at the slaughterhouse [
3].
M. haemolytica can be isolated from pneumonic lungs and from the lung of healthy lambs, with isolates retrieved from both kind of clinical presentations showing a similar genetic background consistent with the idea that individual cases of pneumonia are due to commensal
M. haemolytica strains reaching lungs from the upper respiratory in the presence of predisposing factors [
3]. Optimized control strategies for pneumonic pasteurellosis in sheep require information about the antimicrobial susceptibility of
M. haemoytica respiratory isolates. However, only a limited number of studies have evaluated the antimicrobial susceptibility in vitro of ovine
M. haemolytica isolates, and most of these included a limited number of strains [
4,
5]. Even though most clinical ovine isolates are usually phenotypically susceptible to antibiotics, the emergence of ovine strains with reduced susceptibility to antimicrobials has been reported [
4,
5]. These potentially resistant bacteria may lead to treatment failures, increased costs of animal production and the dissemination of antimicrobial-resistant genes to other bacteria and possibly the environment [
6]. Nevertheless, due to the absence of clinical efficacy studies considering the dosing and route of administration of antimicrobial agents used in veterinary medicine to treat animal diseases for multiple pathogen-drug combination, the clinical significance of in-vitro resistance (or reduced susceptibility) in terms of risk of potential therapeutic failures is difficult to ascertain [
7].
M. haemolytica associated with bovine respiratory disease (BRD) harbouring multiresistance-mediating integrative and conjugative elements (ICEs) have been identified in North America and in European countries [
6,
8]. However, very little knowledge exists about the genetic mechanisms conferring antimicrobial resistance in
M. haemolytica isolates from sheep in spite of its emerging nature. In this context, it is necessary to generate more information regarding the frequency of antimicrobial resistance in isolates belonging to this pathogen originating from different livestock animals and countries [
1]. Thus, the current study aims to investigate the levels of antimicrobial resistance of
M. haemolytica isolated from the lungs of slaughtered sheep and the genetic resistant mechanism involved.
4. Discussion
M. haemolytica is a ruminant-specific pathogen associated with pneumonic pasteurellosis in sheep, a relevant disease due to the important economic losses that generates [
2]. It is generally accepted that the mere presence of
M. haemolytica in lungs is not sufficient to produce lesions [
2] and that individual cases of pneumonia are associated with commensally
M. haemolytica strains of the upper respiratory tract that reach lungs following predisposing factors [
3]. Antimicrobial surveillances provide relevant information necessary to gain knowledge about the epidemiology and empirical treatment options. In this study, we have investigated the antimicrobial susceptibility of 256
M. haemolytica isolates recovered at slaughter from ovine lungs with and without pneumonic lesions. Moreover, we have also determined the genetic antimicrobial mechanisms of resistance in a selection
M. haemoytica resistant to macrolides.
Overall, most ovine
M. haemolytica isolates investigated in this study were classified as susceptible to most (10 of 12) antimicrobials tested, with resistance rates between 0% and 1.2% (
Table 2). These results agree with the high susceptibility of ovine
M. haemolytica isolates to most antimicrobials observed in different studies [
4,
5]. Despite this overall high susceptibility to most antimicrobials, moderate resistance rates were observed for TET (
Table 1). Although not statistically significant (P > 0.05), the frequency of resistance to TET was higher among isolates from lungs with pneumonic lesions (5.9% and 1.1%, respectively;
Table 1). Tetracyclines are among the most frequently antimicrobials used in animals [
22]. Therefore, these resistance rates may reflect the predominance of their use in sheep farming. The
tetH tetracycline-resistance gene, encoding an energy-dependent membrane-associated protein that exports tetracyclines out of the cell [
23], was found in six of the 10 sequenced strains, of which only two were resistant according to the disk diffusion test. When considering the results from the microdilution test (only available for six of the 10 sequenced strains), two of the three strains carrying the gene were classified as resistant (
Table 3). This gene was identified as the predominant
tet gene in
Pasteurellaceae isolates of bovine and swine origin in North America in the nineties [
23] and was more recently described in phenotypically resistant
M. haemolytica isolates retrieved from cattle in the United States [
24]. It was also the most common resistance gene found using metagenomics in nasopharyngeal samples from chronically ill feedlot cattle in Canada [
25], suggesting it may play a significant role in the occurrence of tetracycline resistance in respiratory pathogens from ruminants. However, the presence of
tetH in
M. haemolytica isolates classified as phenotypically susceptible to certain tetracyclines (chlortetracycline) based on clinical breakpoints, as found in this study, has been also previously described, further highlighting the complexities of predicting resistance phenotypes from genetic data in this bacterial species [
26].
The most striking result was the very high resistance rate to TYLO found in this study based on the breakpoint used (89.1 %;
Table 1). This antimicrobial has been classified by WHO as critically important with the highest priority for human medicine [
27], and therefore the spread of genetic determinants mediating to this antimicrobial may represent a serious concern. Similar levels of TYLO resistance have been observed in other respiratory pathogens such as
Pasteurella multocida in sheep and in pigs [
28,
29] in Spain. It has been suggested that this low degree in vitro susceptibility of these bacteria to this macrolide could be due to a long-term exposure to that agent given that it was used at subtherapeutic doses as growth promotor until its ban in 1999 in Spain [
29]. The acquire resistance to macrolides in members of the family
Pasteurellaceae have been associated with the presence of the macrolide resistance gene coding for an efflux pump (
msrE), genes that cause methylation of the ribosomal target (
ermA, ermC and erm42) and the gene that codes for a phosphorylase-inactivating enzyme (
mphE) [
30]. In addition, rRNA mutations that confer resistance to macrolides have been described in field isolates of
M. haemolytica and
P. multocida in cattle [
19] and
H. parasuis in pigs [
30]. However, no genetic determinants (antimicrobial resistance genes or point mutations) conferring resistance to macrolides were identified in any of the isolates sequenced in this study regardless of their TYLO resistance phenotype (
Table 3). Furthermore, no difference in treatment efficacy when using TYLO or tulathromycin in cattle from which TYLO-resistant (and tulathromycin-susceptible)
Pasteurellaceae isolates had been retrieved was previously reported, indicating a lack of direct correlation between in-vitro resistance and therapeutic success [
31]. Previous studies have also reported very high MIC values in
M. haemolytica isolates retrieved from goats [
32] and cattle [
33] while also being predominantly susceptible to other macrolides (tulathromycin, tilmicosin) and/or not harboring known genetic determinants conferring resistance mechanisms to this antimicrobial class. Altogether, these results suggest that
M. haemolytica antimicrobial susceptibility test results for tylosin should be interpreted with care in the absence of well-established epidemiological and clinical breakpoints, since lack of correlation between genetic and phenotypic resistance data observed here and elsewhere could be derived from the presence of still unrecognized resistance mechanisms but also from inappropriate breakpoints [
26].
Mobile genetic elements are known to play a major role in the dissemination of antimicrobial resistance in members of the
Pasteurellaceae family [
6]. In this study, we identified the sequence of a short (~5,300 bp) plasmid carrying both
sul2 and
strA resistance genes (
Table 3). The co-occurrence of these two genes together in plasmids carried by clinical
Pasteurellaceae strains has been already described in isolates retrieved from cattle and swine in several countries [
34,
35] including Spain [
36]. The plasmid retrieved here had a high level of identity with a plasmid sequence originally described in a
P. multocida strain from a diseased pig in China [
21]. Even though the presence of the
strA and
sul2 genes was not associated with increased MIC values to the sulfonamides and aminoglycosides included in the antimicrobial susceptibility testing performed in this study, additional studies to assess their possible role conferring resistance to members of this antimicrobial families are needed.
Author Contributions
Conceptualization, D.C., A.I.V and J.F.F.-G.; methodology, J.A., D.C., A.I.V, L.T.F., M.H. and J.F.F.-G.; software, M.H.; validation, J.A., D.C., A.I.V and J.F.F.-G.; formal analysis, J.A., D.C., A.I.V and J.F.F.-G.; investigation, C.E.P.J., J.M.C.B., L.T.F., M.H.; resources, D.C and J.A.; data curation, D.C and J.A.; writing—original draft preparation, A.I.V. and J.A; writing—review and editing, J.A., D.C., A.I.V, and J.F.F.-G.; visualization, J.A. and D.C; supervision, D.C.; project administration, D.C.; funding acquisition, L.D., J.A. and D.C. All authors have read and agreed to the published version of the manuscript.