Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Arcanobacterium pyogenes, Mycoplasma bovis, and most recently, Bibersteinia trehalosi ,are associated with severe bacterial pneumonia frequently seen in dairy calves (enzootic pneumonia) and in feedlot cattle (shipping fever), says Tony Confer, DVM, MS, PhD, Dipl. ACVP, Oklahoma State University. All of these bacteria are ubiquitous in the cattle population as commensals in the nasopharynx and, following stress or viral infection, can proliferate and be inhaled into the lungs. Each has its own cadre of known virulence factors such as adhesins, toxins, and enzymes that enhance its ability to colonize, cause tissue destruction, and incite an intense inflammatory response. 

What follows are some of the gross lesions associated with these pathogens:

• Mannheimia haemolytica-induced pneumonia is characterized by acute cranioventral fibrinous to fibrinopurulent pleuropneumonia.
• A Pasteurella multocida lesion is a typical cranioventral bronchopneumonia and has been characterized simply as bronchopneumonia or as a bronchopneumonia with various descriptive modifiers including acute fibrinosuppurative, subacute to chronic fibrinopurulent, fibrinous to fibrinopurulent, suppurative, and fibrino-necrotizing.
• Gross lesions of acute fulminating pneumonia due to Histophilus somni are similar to those seen with M. haemolytica infection, i.e. a cranioventral fibrinous pleuropneumonia with hemorrhage and coagulation necrosis that can involve entire lobules. Pulmonary vasculitis is often seen, and those vessels may contain fibrin thrombi. 
• Lesions attributable to Mycoplasma bovis are a cranioventral caseonecrotic bronchopneumonia that may have abscesses, bronchiectasis, and sequestration. Arthritis may accompany respiratory disease.
• Pulmonary lesions ascribed to Arcanobacterium pyogenes are primarily severe abscesses within areas of chronic bronchopneumonia or chronic pleuropneumonia. These abscesses are typically characterized by liquefactive necrosis surrounded by thick fibrous connective tissue band, whereas they are often larger and less caseous than M. bovis-induced lesions. 
• Bibersteinia trehalosi is primarily a sheep pathogen, especially associated with septicemia and has been associated with severe pneumonia in Bighorn sheep and domestic sheep.