Classification of Avian Pathogenic Escherichia coli by a Novel Pathogenicity Index Based on an Animal Model

Abstract

Background: Avian Pathogenic Escherichia coli is the main agent of colibacillosis, a systemic disease that causes considerable economic losses to the poultry industry. In vivo experiments are used to measure the ability of E. coli to be pathogenic. Generally, these experiments have proposed different criteria for results interpretation and did not take into account the death time. The aim of this study was to propose a new methodology for the classification of E. coli pathogenicity by the establishment of a pathogenicity index based in the lethality, death time and the ability of the strain to cause colibacillosis lesions in challenged animals.Materials, Methods & Results: A total of 293 isolates of E. coli were randomly selected to this study. The strains were isolated from cellulitis lesions, broiler bedding material or respiratory diseases and were previously confirmed through biochemical profile. The bacterial isolates were kept frozen at -20°C. The strains were retrieved from stocks and cultured in brain-heart infusion broth overnight at 37°C to obtain a final concentration of 109 UFC/mL. A total of 2940 one-dayold chicks from commercial breeding hens were randomly assigned to groups containing 10 animals and each group was subcutaneously inoculated in the abdominal region with 0.1 mL of the standard inoculum solution containing each of the strains. A control group of 10 broilers were inoculated with 0.1 mL of brain-heart infusion broth by the same route. The chicks were kept for seven days. They were observed at intervals of 6, 12 and 24 h post-inoculation during the first days. From the second day on, the chicks were observed at intervals of 12 h. According to the death time and to the scores of each lesion (aerosaculitis, pericarditis, perihepatitis, peritonitis and cellulitis), a formula to determine the Individual Pathogenicity Index was established. A value of 10 was established as the maximum pathogenicity rate for an inoculated bird. From this rate, 5 points corresponded to scores for gross lesions present at necropsy. For each lesion present, it represents 1 point. The remaining 5 points corresponded to the death time. To obtain the death time value, an index of 1, corresponding to the maximum value assigned to a death on the first day, was divided by the number of days that the birds were evaluated, resulting in a value of 0.1428, which corresponded to a survival bonus factor. It was possible to classify E. coli strains into four pathogenicity groups according to the pathogenicity index: high pathogenicity (pathogenicity index ranging from 7 to 10), intermediate pathogenicity (pathogenicity index ranging from 4 to 6.99), low pathogenicity (pathogenicity index ranging from 1 to 3.99) and apathogenic (pathogenicity index ranging from 0 to 0.99). The analysis of the strains according to their origin revealed that isolates from broiler bedding material presented a lower pathogenicity index.Discussion: It is possible that the source of isolation implies in different results, depending on the criteria adopted. This data reinforces the importance of use a more accurate mathematical model to represents the biological phenomenon. In the study, all avian pathogenic Escherichia coli strains were classified based on a pathogenicity index and the concept of the death time represents an interesting parameter to measure the ability of the strain to promote acute and septicemic manifestation. The use of a support method for poultry veterinary diagnostic accompanying the fluctuation of the bacteria pathogenicity inside the farms may indicate a rational use of antimicrobial in poultry industry.

Avian pathogenic Escherichia coli (APEC) and uropathogenic Escherichia coli (UPEC): characterization and comparison

“…An in vivo pathogenicity index (PI) for APEC strains was previously determined by our group [35]. Briefly, a group of 10 one-day-old chicks was intraperitoneally inoculated with 0.1 mL of a bacterial inoculum containing 10 8 CFU/mL.…”

Establishment of a Pathogenicity Index for Mice to Pasteurella multocida Strains Isolated from Poultry and Swine

“…The IPI was based on the study described by Souza et al (2016) and Pilatti et al (2016) with several modifications. The IPI corresponds to the pathogenicity index (PI) for each animal in each group inoculated with a strain of P. multocida and was calculated on a scale between 0 and 10.…”

Classification of Avian Pathogenic Escherichia coli (APEC) and Human Uropathogenic Escherichia coli (UPEC) in Phylogenetic Groups and Association with Pathogenicity In Vivo

“…O grupo filogenético de cada cepa APEC foi associado aos índices de patogenicidade in vivo obtidos através da inoculação experimental de pintos comerciais com 01 dia de idade [20]. Neste trabalho prévio, 0,1 mL de um inóculo bacteriano contendo 10 8 UFC/mL foi inoculado através da via intraperitoneal.…”

“…As aves foram avaliadas quanto à presença ou ausência de cinco lesões macroscópicas e quanto ao tempo de morte por um período de 07 dias. Desta forma, as cepas foram classificadas em índices de patogenicidade (IP) em uma escala de 0 a 10 [20].…”

“…A diminuição da resistência do hospedeiro devido à imunodepressão resulta em infecções que não são restritas a um único patotipo [3]. Assim, devido às variações ambientais e intrínsecas das cepas quanto à capacidade de causar lesões nos hospedeiros, experimentos in vivo para avaliar a patogenicidade da bactéria em aves tem sido desenvolvidos [15,17,20].…”