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)
