An overview of calf diarrhea - infectious etiology, diagnosis, and intervention

Cho, Yong-il; Yoon, Kyoung-Jin

Fecal bacteria culturing is a commonly used laboratory method for isolating and identifying bacterial pathogens in feces and intestinal contents. Salmonella spp., E. coli K99 + , and C. perfringens are primary bovine enteric pathogens [39, 62] . In order to prevent any cross-contamination or loss of viability, feces should be collected directly from diarrheic calves by either rectal swabs or rectal stimulation. Once collected, the fecal samples should be stored in a transport medium or special stool container in a cooler or on ice before submission to a diagnostic lab. To examine anaerobic bacteria-like C. perfringens, fecal samples must be immediately stored in a pre-reduced (i.e., oxygen-free) transport medium if available. Blood agar plates, MacConkey agar plates, MacConkey agar with sorbitol, Hektoen enteric (HE) plates, and xylose lysine desoxycholate (XLD) plates are used for bacterial culture [23, 99] . Several kinds of enriched and selective media such as brain heart infusion (BHI) broth (a highly nutritious medium for general bacterial culture) and tetrathionate broth (for Salmonella spp.) are employed for growing and identifying certain bacterial pathogens. Blood agar is most commonly used because the majority of bacteria can grow on this medium. MacConkey agar is selectively used to culture Gram-negative bacilli that are commonly present in the gastrointestinal tract and differentiate bacteria that ferment lactose. Sorbitol-MacConkey agar can help distinguish nonpathogenic E. coli from E. coli O157:H7 which cannot ferment sorbitol [33] . Salmonella spp. are typically cultured from fecal samples using Samonell-Shigella agar, bismuth sulfite agar, HE medium, brilliant green agar, and XLD agar [138] . For C. perfringens culturing, thioglycolate broth growth medium is commonly used. Culturing usually takes 2 days at 36 o C under anaerobic conditions [39] . Colony morphology (e.g., shape, surface, and elevation of colonies on the agar plates), physical characteristics of the bacteria (e.g., aerobe, anaerobe, or microaerophile), microscopic features (e.g., rods, cocci, or coccobacilli), and biochemical tests (e.g., ones that confirm fermentation, gelatin or urea utilization; indole, oxidase, or catalase production, etc.) are then used to characterize and identify the isolated bacteria. Slow turnaround of the results (growth and identification can take 24∼72 h) is a disadvantage of bacterial culture tests although the turnaround can vary depending on culture methods and diagnostic instrumentation. In some cases, further immunological testing (e.g., an agglutination test) is required for the identification (e.g., for E. coli K99 + ) [18] or serotyping (e.g., for Salmonella spp.) of bacteria [73] . A nucleic acid-based assay is also required for typing (e.g., for C. perfringens toxin type) [50] .