Bacteria are the major cause of severe sepsis. They can be classified in various ways, and the most common are listed in the table to the right.

Gram positive

• These organisms have a cell wall that contains peptidoglycans, lipoteichoic acid and polysacharides.
• Produce exotoxin.
• Cocci: staphylococci, enterococci and streptococci.
• Bacilli: bacillus, cornybacterum, mycobacterium and clostridium.

Gram negative

• Much thinner peptidoglycan layer. No teichoic or lipoteichoic acids.
• Have an additional outer cell wall which contains endotoxin.
• Cocci: neisseria.
• Bacilli: enterobacteriaceae, acinetobacter, pseudomonas, campylobacter, haemophilus, legionella and chlamydia.

There are also atypical bacteria. They include:

• Legionella.
• Mycoplasma.
• Chlamydia.
• Rickettsia.
• Borrelia.