3.04 Gram Negative Bacteria

Microbiology review for the USMLE Step 1 Exam.

• Gram negative bacteria are characterized by a thin cell wall surrounded by a lipid outer membrane
• They do not retain the violet gram stain and appear pink after staining
• Gram negative bacteria can be classified by shape: cocci, coccobacilli, bacilli, and spirals
• Gram negative cocci include Neisseria species, which can be further categorized as maltose fermenters or non maltose fermenters
• Gram negative coccobacilli include: Bordetella pertussis, Haemophilus influenzae, Pasteurella, Francisella tularensis, Brucella, and Acinetobacter
• Gram negative bacilli can be divided into lactose fermenting and non lactose fermenting categories
• Lactose fermenting gram negative bacilli include: Escherichia coli, Klebsiella, Enterobacter, Citrobacter, and Serratia
• Non lactose fermenting gram negative bacilli can be further divided into oxidase positive and oxidase negative
• Oxidase positive non lactose fermenting gram negative bacilli include Pseudomonas
• Oxidase negative non lactose fermenting gram negative bacilli include Shigella, Yersinia, and Salmonella
• Gram negative spirals can be divided into oxidase positive and oxidase negative categories
• Oxidase positive gram negative spirals include Vibrio species, Helicobacter pylori, and Campylobacter jejuni
• Oxidase negative gram negative spirals include Borrelia burgdorferi (cause of lyme disease) and Treponema pallidum (cause of syphilis)