Following exposure to irradiation the affected subject is increased susceptibility to systemic infection (sepsis). The infections can be endogenous originating from the oral and gastrointestinal bacterial flora, and exogenous originating from a breached skin and environment following trauma or burn (including severe radiation burn). (Brook et al. Effect of radiation dose on the recovery of aerobic and anaerobic bacteria from mice. Can J Microbiol. 1986; 32: 719. Brook & Ledney, 1992.). The presence of combined injury ( exposure to radiation plus trauma or radiation) increases the likelihood of sepsis. (Palmer et al. Development of a combined radiation and burn injury model. J Burn Care Res. 2011; 32:317)
The organisms causing endogenous infections are Gram negative bacilli such as Enterobacteriacae (i.e. Escherichia coli, Klebsiella pneumoniae, Proteus spp. ), Pseudomonas aeruginosa, and Enterococcus spp.( of gastrointestinal origin) and Streptococcus spp. (of oral cavity source). ( Brook & Ledney , 1992)
Exposure to higher doses of radiation is associated with systemic anaerobic infections due to Gram negative bacilli and gram positive cocci. Fungal infections can also emerge in those that fail antimicrobial therapy and stay febrile for over 7-10 days.
Exogenous infections can be caused by organisms that colonize the skin such as Staphylococcus aureus or Streptococcus spp. and organisms that are aquired from the environment after injury, laceration or burn such as Pseudomonas spp. Infection caused by Pseudomonas is very rapidly fatal. ( Brook & Ledney. Quinolone therapy in the prevention of endogenous and exogenous infection after irradiation. J Antimicrob Chemother. 1994; 33:777.)