Gram Negative Bacteria: Classification, Pathogens, and Clinical Relevance

Gram Negative Bacteria

Bergey’s Classification Criteria

Bergey’s Manual is a foundational reference for bacterial taxonomy, using a combination of phenotypic and metabolic characteristics to classify bacteria. The following criteria are essential for identifying and classifying Gram negative bacteria:

• Cell shape and arrangement: Morphological features such as cocci, bacilli, spirilla, and their arrangements (chains, clusters, pairs).

• Gram stain reaction: Gram negative bacteria appear pink/red after Gram staining due to their thin peptidoglycan layer and outer membrane.

• Oxygen requirements: Classification as obligate aerobes, obligate anaerobes, facultative anaerobes, or microaerophiles.

• Motility: Presence or absence of flagella or other motility structures.

• Nutritional requirements: Ability to utilize various carbon and energy sources.

• Metabolic properties: Biochemical reactions such as fermentation, enzyme production, and metabolic byproducts.

Obligate Intracellular Bacteria

Rickettsia rickettsii

• Disease: Rocky Mountain Spotted Fever

• Transmission: Tick vector (Dermacentor species)

• Symptoms: Fever, headache, characteristic rash on palms and soles

• Diagnosis: Serology (antibody detection), tissue biopsy

• Treatment: Doxycycline (antibiotic of choice)

Chlamydia trachomatis

• Disease: Nongonococcal urethritis, pelvic inflammatory disease (PID), trachoma (leading cause of preventable blindness)

• Transmission: Sexual contact, perinatal (mother to child during birth)

• Diagnosis: DNA probes, immunofluorescence assays

• Treatment: Doxycycline or Azithromycin

Spirochetes

Treponema pallidum

• Disease: Syphilis (progresses through primary, secondary, latent, and tertiary stages)

• Diagnosis: Non-treponemal tests (RPR), treponemal tests (FTA-ABS), darkfield microscopy

• Treatment: Penicillin (drug of choice)

Borrelia burgdorferi

• Disease: Lyme Disease

• Transmission: Deer tick (Ixodes species)

• Diagnosis: ELISA (screening), Western blot (confirmation)

• Treatment: Doxycycline

Curved / Helical Gram Negative Rods

Vibrio cholerae

• Disease: Cholera

• Symptoms: Profuse watery diarrhea ("rice-water stools"), severe dehydration

• Treatment: Fluid and electrolyte replacement, Doxycycline

Campylobacter jejuni

• Disease: Gastroenteritis (most common bacterial cause in developed countries)

• Growth: Grows at 42°C on CAMPY agar

• Treatment: Usually self-limiting; antibiotics in severe cases

Helicobacter pylori

• Disease: Peptic ulcers, gastric carcinoma

• Key Feature: Urease positive (breaks down urea to ammonia, neutralizing stomach acid)

• Diagnosis: Urea breath test

• Treatment: Combination therapy (bismuth + antibiotics)

Aerobic Gram Negative Rods

Pseudomonas aeruginosa

• Opportunistic pathogen: Causes infections in immunocompromised hosts (e.g., burn patients, cystic fibrosis)

• Pigment: Produces blue-green pigment (pyocyanin)

• Biochemical: Non-lactose fermenter

• Treatment: Quinolones (e.g., ciprofloxacin)

Legionella pneumophila

• Disease: Legionnaires’ disease (severe pneumonia)

• Transmission: Inhalation of water aerosols (e.g., air conditioning systems)

• Diagnosis: Charcoal yeast extract agar, urine antigen test

• Treatment: Azithromycin

Enteric Gram Negative Rods (Facultative Anaerobes)

These bacteria are part of the family Enterobacteriaceae and are commonly found in the intestinal tract. They are identified by their ability to ferment glucose and other biochemical tests.

Organism	Key Features	Diseases	Treatment/Notes
Escherichia coli	Lactose fermenter, indole positive	UTI (most common cause), EHEC O157:H7 causes Hemolytic Uremic Syndrome	Supportive; antibiotics in severe cases
Proteus mirabilis	Swarming growth, urease positive	UTIs, kidney stones	Antibiotics based on sensitivity
Salmonella enterica	Non-lactose fermenter	Gastroenteritis	Supportive; antibiotics in severe cases
Salmonella typhi	Systemic infection	Typhoid fever	Antibiotics (e.g., fluoroquinolones)
Shigella	Non-motile	Bacillary dysentery (bloody diarrhea)	Supportive; antibiotics in severe cases
Yersinia pestis	Bipolar staining	Plague (bubonic, septicemic, pneumonic)	Tetracycline

Fastidious Gram Negative Rods

Haemophilus influenzae

• Growth Requirements: Requires X (hemin) and V (NAD) factors for growth

• Diseases: Epiglottitis, meningitis (especially in children)

• Prevention: Hib vaccine

Bordetella pertussis

• Disease: Whooping cough (pertussis)

• Diagnosis: Bordet-Gengou agar, PCR

• Prevention: DTaP vaccine

Gram Negative Diplococci

Organism	Diseases	Diagnosis	Treatment/Prevention
Neisseria gonorrhoeae	Gonorrhea	Growth on Thayer-Martin agar	Cephalosporins
Neisseria meningitidis	Meningococcal meningitis	CSF Gram stain, latex agglutination	Penicillin

Summary Table: Key Gram Negative Pathogens

Group	Representative Genera	Key Diseases	Diagnosis	Treatment/Prevention
Obligate Intracellular	Rickettsia, Chlamydia	Rocky Mountain Spotted Fever, Trachoma, PID	Serology, DNA probes	Doxycycline, Azithromycin
Spirochetes	Treponema, Borrelia	Syphilis, Lyme Disease	Serology, ELISA, Western blot	Penicillin, Doxycycline
Curved Rods	Vibrio, Campylobacter, Helicobacter	Cholera, Gastroenteritis, Peptic Ulcers	Culture, Urea breath test	Fluids, Antibiotics
Aerobic Rods	Pseudomonas, Legionella	Pneumonia, Opportunistic infections	Culture, Urine antigen	Quinolones, Azithromycin
Enteric Rods	Escherichia, Salmonella, Shigella, Yersinia	UTI, Gastroenteritis, Plague	Culture, Biochemical tests	Supportive, Antibiotics
Fastidious Rods	Haemophilus, Bordetella	Meningitis, Whooping cough	Special media, PCR	Vaccines, Antibiotics
Diplococci	Neisseria	Gonorrhea, Meningitis	Culture, CSF analysis	Cephalosporins, Penicillin

Additional info: The above notes expand on the original study guide by providing definitions, clinical context, and summary tables for easier comparison and review. Where only brief points were given, academic explanations and examples have been added for clarity and completeness.