Histophilus somni is a non-motile, gram-negative, rod or coccobacillus shaped, facultative anaerobe bacterial species belonging to the family Pasteurellaceae. Prior to 2003, it was thought Haemophilus somnus, Histophilus ovis, and Histophilus agni were three different species, but now are all classified as Histophilus somni. Histophilus somni is a commensal bacteria of mucous membranes of the upper respiratory tract and reproductive tract with a global prevalence and is found in cattle and other small ruminants. Histophilus somni is also a known causative agent that is a part of the Bovine Respiratory Disease (BRD) complex, which typically involves multiple pathogens residing together in biofilm environments. Histophilus somni may also cause Histophilosus symptoms and clinical presentation will depend on the tissue affected. When disease does occur, it can be difficult to catch in time and is often diagnosed post mortem. This means that treatment often involves metaphylactic mass treatment or no treatment at all. This organism is more fastidious than others and requires knowledge for sample collection, storage and culture. Genomic studies related to this bacteria have enabled scientists to pinpoint antibiotic resistance genes.
Description Histophilus somni is a member of the Pasteurellaceae family and was first isolated from cattle in 1956. Histophilus somni is a gram-negative, rod or coccobacillus shaped bacteria that does not express pili or flagella making it non-motile. Many species of the Pasteurellaceae family are encapsulated; however, based on electron microscopy and ruthenium red staining results H. somni is not encapsulated. Prior to 2003 Histophilus somni was considered to be three different species Haemophilus somnus, Histophilus ovis, and Histophilus agni. Histophilus somni is a normal part of mucosal microbiota in the upper respiratory and genital tract of bovine and ovine animals, yet, under the right conditions is one of multiple bacteria responsible for causing bovine respiratory disease. Under normal conditions BRD complex bacteria symbiotically reside together in biofilm environments. The biofilm protects H. somni from harmful substances while allowing co-existence with the host by down regulating virulence factor productions. Although H. Somni is known to survive the harsh intracellular environments of host phagocytic cells, the bacteria is considered fragile in the external environment and does not survive well outside of the host.
Culture and biochemistry
Culture and identification Histophilus is a fastidious bacterium and has some special requirements for culturing. A differentiating factor between Histophilus somni and the Haemophilus species is that Histophilus species are able to grow in the absence of Factor X (heat stable hemin) and Factor V (NAD) although their growth is increased in the presence of these factors. Histophilus grows best on chocolate agar at 37 °C with 5-10% CO2 and is unable to grow on MacConkey agar. Cultures have been developed in hopes to make culturing Histophilus easier, but trials are usually unsuccessful. Histophilus can be grown in Columbia broth or brain heart infusion broth with the supplementation of thiamine monophosphate. Culturing in a broth before plate culturing will help isolate Histophilus from other commensal bacteria at the sample site. The colonies are quite small and usually only reach a size of 1-2mm. When grown on blood agar, Histophilus will have a clear areas around the colonies. The dew-drop shaped colonies grown on chocolate agar are tinged yellow which is a distinguishing factor of these colonies. Because Histophilus is fragile outside the host, care must be taken when collecting samples for laboratory diagnosis. Samples can try out very easily and should be cultured as soon as possible. The best storage method for samples to stay viable is for deep freezing below -60 degrees Celsius. The best way to identify H. somni is through 16S rRNA gene PCR amplification. Microagglutination assays have been developed for identification in cattle however many animals have positive cross-reactive antibodies without having had the infection. Methods have been developed for the differentiation between ovine and bovine strains of H. somni, including restriction enzyme analysis. Microscopy can be used for identification when bacteria are stained with fluorescent antibody stain.
Biochemistry Histophilus species can be difficult to identify with biochemical reactions as many of the tests used for identification do not have consistent results. Tests consistent throughout Histophilus colonies are oxidase and catalase tests, with oxidase having positive results and catalase being negative. Histophilus does not exhibit consistent hemolysis. Their ability to reduce nitrogen allows them to be facultative anaerobes and their growth on culture will increase with an increased atmospheric CO2. Histophilus has the ability to ferment glucose, has variable results with lactose and mannitol and are unable to ferment sucrose. Antimicrobial testing has demonstrated that H. somni is still susceptible to most antimicrobials, though some strains have shown resistance to tetracycline.
Genetics In 1995 Haemophilus influenzae was genetically sequenced, a close relative to Histophilus somni and the first free living organism to have its complete genome sequenced. Due to the economic importance from production losses the genomes of both Histophilus somni pneumonia strain 2336 (2,263,857 base pairs with 1,980 protein coding genes) and preputial strain 129Pt (2,007,700 base pairs with 1,792 protein coding genes) have since been sequenced. The genome studies of Histophilus somni strains have identified specific markers encoding tetracycline resistance, and virulence factors, while allowing a better understanding of the role of horizontal gene transfer in the evolution of these strains Plasmid-borne antimicrobial resistance is an important topic in modern microbiology and occurs commonly in members of the Pasteurellaceae family. Histophilus somni with tetracycline resistant plasmids have been cultured from nasal swabs of calves. Using plasmid profiling as a way to identify isolated Histophilus somni from field samples gives veterinarians the ability to practice antimicrobial stewardship and reduce the impact of antimicrobial resistance.
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