Gram Stain Interpretation

Expansion

Gram positive cocci

• Clusters: Staphylococcus. Coagulase positive is S. aureus; coagulase negative is S. epidermidis and relatives
• Chains or pairs: Streptococcus and Enterococcus. Alpha haemolytic includes S. pneumoniae (optochin sensitive) and viridans; beta haemolytic includes group A S. pyogenes and group B S. agalactiae

Gram positive bacilli: Listeria, Clostridium, Bacillus, Corynebacterium, Actinomyces, Nocardia.

Gram negative cocci: Neisseria meningitidis and N. gonorrhoeae, and Moraxella.

Gram negative bacilli

• Enterobacterales: Escherichia coli, Klebsiella, Proteus, Enterobacter, Salmonella, Shigella
• Non-fermenters: Pseudomonas aeruginosa, Acinetobacter
• Coccobacilli: Haemophilus influenzae, Bordetella, Brucella
• Curved: Vibrio, Campylobacter, Helicobacter

Organisms that do not Gram stain usefully

• Mycobacteria: no peptidoglycan exposure, so they need Ziehl-Neelsen or auramine, and appear as acid fast bacilli
• Mycoplasma: no cell wall at all, which is why beta lactams do not work
• Chlamydia, Rickettsia, Legionella (poorly), Treponema (dark ground or serology), and fungi, which need silver, PAS or calcofluor

Practical value: a Gram stain on cerebrospinal fluid, joint fluid, pleural fluid or pus gives an answer within the hour and directs empirical therapy long before culture. Gram positive diplococci in the CSF of an adult means pneumococcus, and Gram negative diplococci means meningococcus.