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Introduction
1. Anatomy
2. Microbiology
2.1 General bacteriology
2.2 Introduction to systemic bacteriology
2.3 Gram positive cocci
2.4 Gram negative cocci
2.5 Gram positive bacilli
2.6 Gram negative bacilli
2.6.1 Introduction
2.6.2 E.coli
2.6.3 Klebsiella pneumoniae
2.6.4 Proteus
2.6.5 Vibrio cholerae
2.6.6 Other Vibrios
2.6.7 Salmonella
2.6.8 Pseudomonas
2.6.9 Minor pathogens
2.7 Other important bacteria
2.8 Virology
2.9 Parasitology
2.10 Mycology
3. Physiology
4. Pathology
5. Pharmacology
6. Immunology
7. Biochemistry
8. Cell and molecular biology
9. Biostatistics and epidemiology
10. Genetics
11. Behavioral science
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2.6.1 Introduction
Achievable USMLE/1
2. Microbiology
2.6. Gram negative bacilli

Introduction

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This group includes a wide variety of clinically important bacteria. They can cause infections ranging from gastroenteritis and pneumonia to hospital-acquired infections. Here, you’ll focus on USMLE-relevant features that help you make the correct diagnosis from a clinical vignette and confidently rule out close differentials.

E.coli
E.coli
NLF versus LF colonies
NLF versus LF colonies

Table 5.1 showing differentiating features of Gram negative bacilli

Bacteria Lactose Fermenter * Motility Oxidase Test Capsule Special Morphological Characters
E.coli Yes Motile Negative Some strains, capsulated will be more virulent.
Klebsiella pneumoniae Yes Non-motile Negative Yes Large, mucoid colonies
Proteus No Swarming motility Negative No Pleomorphic, coccobacilli, fishy or seminal smell, urease positive
Enterobacter Motile Negative Yes Mucoid colonies
Serratia Yes, slowly Motile Negative Yes Red pigment
Citrobacter Yes, slowly Motile Negative Maybe present
Vibrio cholerae Yes, slowly Darting motility Positive No Curved, S shaped ##
Vibrio parahaemolyticus No Motile Positive Yes Straight or curved, need high salt conc.
Vibrio vulnificus Yes Motile Positive Yes Straight or curved, need high salt conc.
Vibrio alginolyticus No Motile Positive Yes Straight or curved, need high salt conc.
Shigella No *** Non-motile Negative No
Salmonella No Motile Negative No Black colonies on some media**
Pseudomonas No Motile Positive No, but produce slime layer and biofilms Produce bluish green pigment, fruity odor

*Lactose fermenters will produce pink colonies on Mac Conkey’s agar. Slow lactose fermenters may initially appear colorless or faint pink on Mac Conkey’s agar, then become more pink over time due to late fermentation of lactose.

**Salmonella produce black colonies on H2S producing media such as TSI and Wilson and Blair media.

***All strains of Shigella are non-lactose fermenters except for Shigella sonnei, which is a late lactose fermenter.

Trick question: which is the other Gram negative, comma or S shaped bacillus? Hint: it is associated with Guillain Barre Syndrome!

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