Staphylococcus aureus is a well-known pathogen that is very opportunistic and commonly causes abscess lesions. It routinely may resist phagocytosis by WBCs due to protein A. Osteomyelitis and arthritis, either hematogenous or traumatic, are commonly caused by S. aureus, especially in children. Salmonella are gram-negative. Staphylococcus saprophyticus is a common skin flora and is usually not pathogenic. Streptococcus pneumoniae is seldom or never involved in osteomyelitis infections, as is true for L. monocytogenes.