Final answer:
Bacteria, specifically Group B streptococcus, is the most common cause of neonatal septicemia, with preventative measures such as maternal antibiotics during labor significantly reducing risk.
Step-by-step explanation:
The most common cause of neonatal septicemia is bacteria. Specifically, Group B streptococcus (GBS), or S. agalactiae, is the encapsulated gram-positive bacterium that often leads to early-onset disease which occurs in the first seven days of life. Babies can become infected by S. agalactiae during childbirth from the mother's vagina. Late-onset neonatal meningitis, which can occur from one week to three months of age, also largely involves this bacterium, although other sources of infection besides the mother can contribute.
Preventative measures include providing intravenous antibiotics to the mother during labor to greatly reduce the incidence of early-onset neonatal meningitis. Diagnosis involves cultures of cerebrospinal fluid (CSF) or blood and treatment is commonly with ß-lactam antibiotics such as intravenous penicillin or ampicillin plus gentamicin. It's critical to understand that untreated neonatal meningitis caused by bacterial infection can lead to severe complications and even death.