Classification of bacteria
• Gram stain
▫Gram positive vs. gram negative
▫Don’t stain (acid fast organisms – mycobacteria, no cell wall – mycoplasma)
•Other factors to considerCan the antibiotic(s) get to the site of infection?
Will the antibiotic(s) be active at the site of infection?
Is the drug bactericidal or bacteristatic?
Are there any potential drug interactions?
▫Oxygen requirements (anaerobic, aerobic)
▫Motility
▫Spore production

Gram positive bacteria
Cocci
>>pairs & chains (S pneumoniae, S pyogenes, Grp B strep, Viridans Strep, enterococci)
>>clusters
>>>>>coagulase + (S aureus)
>>>>>coagulase - (seen more in neonates)
Bacilli
>>aerobic or facultative anaerobes (Listeria spp)
>>anaerobes
>>branching

Gram negative bacteria
Bacilli
>>Enterobacteriaceae (E. coli, Klebsiella spp., Enterobacter spp., Salmonella spp., Shigella spp.)
>>Various (respiratory ones) (Haemophilus influenzae, Bordetella pertussis)
>>non-fermenters (pseudomonas spp)
>>anaerobes
Cocci
>>Neisseria gonorrhoeae (commonly in babies w/ conjunctivitis), Neisseria meningitidis, Moraxella catarrhalis

Principles of antibiotic use and spectrum of activity
Selecting the correct antibiotic - Think bug, drug, host
>>Bug - What are the likely pathogens? What is the likelihood of antibiotic resistance?
>>Drug - Can the antibiotic(s) get to the site of infection? Will the antibiotic(s) be active at the site of infection? Is the drug bactericidal or bacteristatic? (eg meningitis vs UTI) Are there any potential drug interactions?
>>Host - Are there any underlying medical or surgical conditions? (eg renal failure, liver dysfn) Is the host immunologically normal? Are there any foreign bodies in place? Does the patient have any drug allergies? How expensive will the treatment course be?

Non-antibiotic considerations
>>Source control – consider the role of surgery (eg need to drain abscess first before abx use)
>>Anatomic and tissue related factors
▫Blood brain barrier
▫Osteomyelitis, sequestrum
▫Vegetations or necrotic tissue (devascularized tissue)
>>Ability of antibiotic to act at site of infection
▫Hard to eradicate organisms from foreign-body material
▫Abscess fluid
Many antibiotics do not penetrate well
Some antibiotics inactive in acid pH (e.g. aminoglycosides)

Cidal versus static
>>Bactericidal antibiotics kill bacteria (Cidal antibiotics preferred for serious life-threatening conditions)
▫Cell wall active agents (beta-lactams, vancomycin)
▫Daptomycin, quinolones, metronidazole, co-trimoxazole
>>Bacteristatic antibiotics inhibit bacterial growth and reproduction without killing them
▫Inhibitors of protein synthesis (macrolides, tetracyclines, clindamycin, chloramphenicol)
▫Sulfonamides, trimethoprim

Penicillin: spectrum of activity
>>Gram positive cocci (streptococci or any relatives/similar ones)
▫Group A, B, C, G streptococci, S. pneumoniae, S. bovis, viridans streptococci, enterococci
>>Gram negative cocci and rods
▫Neisseria gonorrhoeae & N. meningitidis, Pasteurella multocida
>>Other
▫Oral anaerobes (non-β-lactamase producing; fusobacteria, peptostreptococcus, prevotella spp., Bacteroides fragilis)
▫Syphilis
▫Selected gram positive bacilli including Clostridium spp., Actinomyces spp., Bacillus spp., C. diphtheriae