Colibacillosis: Dr. Terezinha Knöbl answers main issues regarding the topic (PART 1)03/05/2022
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As avian colibacillosis is a very critical disease with a great economic impact on the production of broilers, we invited an expert on the subject to answer the main questions on the subject.
With extensive experience in avian diseases and infections caused by enterobacteria, Dr. Terezinha Knöbl works at the Avian Medicine Laboratory of the Pathology Department of Faculdade de Medicina Veterinária e Zootecnia of Universidade de São Paulo (USP).
In part 1 of our interview, she detailed the problems caused by the pathogenic strains of Escherichia coli (the APECs). Now, we reserve a space for those who want to delve further into the subject.
In this article, Dr. Terezinha addresses issues related to the pathogenic potential of E. coli strains, virulence markers, resistance genes and talks about how to isolate an APEC in routine examination. Check out part 2 of this conversation!
Interview with Dr. Terezinha Knöbl
1) Biocamp – What are the virulence markers of an E. coli strain that give us clues about its pathogenic potential?
Dr. Terezinha Knöbl – There are several protocols to establish the potential for pathogenicity. However, APEC is a very diverse pathotype, with more than 25 distinct markers. The most used protocol for diagnosing virulent strains is that of Johnson et al. (2008), who researched the presence of 5 virulence factors considered minimal predictors:
Additionally, studies that correlate the presence and absence (by deletion) of virulence genes with pathogenicity in 1-day-old embryos or chicks highlight the importance of genes related to iron capture, the presence of the ColV plasmid, the thermosensitive adhesin tsh and the iss protein (increased survival serum). Some virulence markers link APEC to other ExPEC pathotypes, such as the usp, papC and cnf genes (common in uropathogenic E. coli) or sfa, ibeA and kps (present in meningitis-associated E. coli). These markers suggest the zoonotic risk of an APEC strain.
2) Biocamp – Knowing that E. coli can be a contaminant of faecal origin of the sample or a secondary agent, how to define if APEC is the etiological agent of a clinical condition?
Dr. Terezinha Knöbl – Some points are essential to consider an APEC as the etiological agent of a clinical condition. Isolation must have been performed from an extraintestinal site and under adequate aseptic conditions. There can be no evidence of fecal contamination in the collection or transport of the sample. The chicken must have lesions suggestive of colibacillosis. Generally, more than one chicken with the same lesions results in the isolation of the same agent.
There are phenotypic models that correlate virulence with pathogenicity, but these models use animals and, for ethical reasons, should only be used in specific situations.
From the point of view of molecular diagnosis, it should be considered as the primary agent when the isolate has APEC virulence factors and belongs to phylogroup G described by Clermont et al (2019). Isolates belonging to phylogroups B2, D or F are also often pathogenic.
In cases of opportunism, in which commensal strains act as secondary agents, the isolates are preferentially classified into groups A or B1. Other forms of classification include serotyping, considering that certain serogroups — such as O78, O2, O1 and O21 — classically refer to cases of avian colibacillosis.
More recently, the MLST technique proved to be useful in the identification of dominant sequences in outbreaks of colibacillosis. APEC isolates have primary participation when the predominant lineage belongs to the group formed by ST117, ST95, ST23, ST140 and ST428/429. Some other STs can also affect birds and are considered potentially zoonotic, such as ST73, ST69 and ST131.
3) Biocamp – Are virulence and resistance genes in E. coli associated?
Dr. Terezinha Knöbl – There is some limitation of bacterial genome size and, in the evolutionary process, the different strains tend to accumulate virulence or resistance repertoires. So, generally speaking, very virulent bacteria tend to be less resistant and very resistant bacteria tend to be less virulent. But there are exceptions.
Some mobile elements such as plasmids and integrons may contain virulence and resistance genes. If there is selective pressure for antimicrobials and the virulence genes are in the same genetic environment, there will be co-selection of virulent and resistant bacteria. So, it all depends on the genetic environment in which these virulence and resistance determinants are hosted.
4) Biocamp – What to do when isolating an APEC in a routine exam? Does that just justify the appearance of some disease? Does it make sense to isolate it from feces or the digestive tract?
Dr. Terezinha Knöbl – In the case of routine APEC isolation, it is important to characterize the agent and monitor the next flocks. The interpretation of this finding should consider the isolation site, since APEC can colonize the intestine of birds without causing enteric disease. Isolation from feces does not confirm the diagnosis of colibacillosis, but points to a risk of horizontal and vertical transmission of the agent. The risk will largely depend on the isolated strain. The isolation of organs such as the liver, heart, air sacs, lung, sinuses, yolk sac, ovarian follicles or bone marrow confirms the diagnosis of colibacillosis (provided that the sample collection has been carried out rigorously, free of fecal contamination). In this case, if the occurrence is not punctual and the cases are associated with impacts resulting from mortality and condemnation at the slaughterhouse, it is necessary to adopt more specific control measures.
5) Biocamp – Since E. coli is a constituent of the normal microbiota of healthy birds, what is the meaning of the isolation of E. coli in the intestines of birds with enteric and respiratory problems?
Dr. Terezinha Knöbl – In general, APEC has little or no participation in enteric processes. These are usually more related to dysbiosis processes, in which there is a quantitative change between different bacterial populations. In these cases, there may be some impact on the zootechnical performance.
In respiratory processes, E. coli strains can act as primary or opportunistic agents of disease, resulting in aerosacculitis and/or sepsis. Primary or secondary participation depends on the virulence repertoire. In general, APEC is considered the primary agent and commensal strains of phylogroup A or B1 are opportunistic.