Small Pleomorphic Gram-Negative Bacilli – Part 1

Vocabulary flashcards covering key organisms, growth requirements, virulence factors, clinical diseases, and laboratory characteristics from the lecture on small pleomorphic gram-negative bacilli (Part 1).

Small Pleomorphic Gram-Negative Bacilli

Group of fastidious, shape-variable, gram-negative rods/coccobacilli that require enriched media and often special growth factors.

Pleomorphism

Ability of a bacterium to alter its size and shape in response to environmental conditions.

Fastidious Microorganism

Bacterium that needs complex nutritional or environmental requirements for growth, making it difficult to culture.

Capnophile

Organism that grows best in an atmosphere with elevated CO₂ levels.

Haemophilus (genus)

Small, non-motile, pleomorphic, gram-negative coccobacilli that are normal upper-respiratory flora and require X and/or V factors.

X Factor (Hemin)

Heat-stable iron-porphyrin (hemin) required by some Haemophilus species for growth.

V Factor (NAD)

Heat-labile cofactor nicotinamide adenine dinucleotide needed by many Haemophilus species.

Haemophilus influenzae

‘Pfeiffer’s bacillus’; major cause of pediatric meningitis and epiglottitis; commonly tested for β-lactamase production.

Pfeiffer’s Bacillus

Alternate name for Haemophilus influenzae.

β-Lactamase

Enzyme that hydrolyzes the β-lactam ring of penicillins/cephalosporins, conferring resistance.

H. influenzae Polysaccharide Capsule

Type-specific capsule (especially type b) that increases invasiveness and severity of infection.

IgA Protease

Virulence enzyme of H. influenzae that cleaves secretory IgA, aiding mucosal colonization.

Encapsulated vs Non-encapsulated H. influenzae

Encapsulated (typeable) strains cause invasive disease; non-encapsulated (non-typeable) strains usually cause local, milder infections.

Epiglottitis

Life-threatening inflammation of the epiglottis commonly caused by encapsulated H. influenzae.

Haemophilus ducreyi

Agent of chancroid; not normal flora; produces painful genital ulcers; requires X factor only.

Chancroid (Soft Chancre)

Painful necrotizing genital ulcer caused by Haemophilus ducreyi; highly communicable STI.

Haemophilus aegyptius

‘Koch-Weeks bacillus’; genetically related to H. influenzae; causes acute purulent conjunctivitis (pink-eye).

Pink-Eye Conjunctivitis

Red, swollen eyes with sticky discharge produced by H. aegyptius infection.

Haemophilus parainfluenzae

V-factor-dependent oral commensal that can cause endocarditis, often affecting the mitral valve.

Satellite Phenomenon

Growth of V-factor-dependent Haemophilus as tiny colonies around NAD-producing organisms (e.g., S. aureus) on blood agar.

Blood Agar vs Chocolate Agar

Blood agar supplies X factor; chocolate agar (heated blood) supplies both X and V factors for Haemophilus growth.

Staphylococcus aureus (V-factor Producer)

Common helper organism used to supply V factor on blood agar for Haemophilus isolation.

HACEK Group

Oral flora gram-negative bacilli (Haemophilus/ Aggregatibacter, Actinomycetemcomitans, Cardiobacterium, Eikenella, Kingella) causing slow endocarditis.

Aggregatibacter (Haemophilus) aphrophilus

Most prevalent HACEK member; V-factor-dependent; causes endocarditis; yellow-granular colonies.

Aggregatibacter actinomycetemcomitans

HACEK species linked to periodontitis; produces collagenase and leukotoxin; star-shaped colonies.

Star-Shaped Colonies

Colony morphology (4–6 pointed center) characteristic of Aggregatibacter actinomycetemcomitans on agar.

Cardiobacterium hominis

HACEK member that infects aortic valve; shows rosette cell arrangement and pits agar surface.

Rosette Formation

Flower-like clustering of gram-negative bacilli seen with Cardiobacterium hominis.

Eikenella corrodens

Chlorine-bleach-odored, agar-corroding bacillus causing cellulitis in drug users and clenched-fist wounds.

Clenched-Fist Wound Infection

Soft-tissue infection of the hand following human bite trauma, often involving Eikenella corrodens.

Kingella kingae

Non-motile HACEK bacillus causing bone and joint infections in children <3 yrs; resists decolorization.

Brucella species

Intracellular, strict aerobes (B. abortus, B. melitensis) acquired from animals/milk; Category B bioterror agents.

Brucellosis

Zoonotic infection with fever, malaise, possible chronicity; transmitted via aerosols, contact, or unpasteurized dairy.

Undulant (Malta) Fever

Characteristic fluctuating fever pattern of sub-chronic brucellosis—normal in morning, spikes later in day.

Bordetella pertussis

Strictly human pathogen causing whooping cough; fastidious aerobic gram-negative coccobacillus.

Whooping Cough (Pertussis)

Highly contagious respiratory illness with paroxysmal coughing followed by inspiratory ‘whoop’.

Mercury Droplet Colonies

Shiny, domed colonies of B. pertussis on Bordet-Gengou or Regan-Lowe agar.

Regan-Lowe Medium

Charcoal, horse-blood, cephalexin transport/enrichment medium for B. pertussis.

Paroxysmal Stage (Pertussis)

Second clinical phase marked by severe, repetitive coughing fits that may cause vomiting.

Francisella tularensis

Tiny, cysteine-requiring gram-negative coccobacillus causing tularemia; Category A bioterror agent.

Tularemia

Zoonotic disease (often from ticks, rabbits) featuring ulceroglandular lesions and systemic symptoms.

Eschar (Tularemia)

Black necrotic ulcer at Francisella inoculation site.

Pasteurella multocida

Mushroom-odor, penicillin-susceptible gram-negative rod from animal bites; oxidase, indole positive.

Mushroom Odor Colony

Distinct earthy smell produced by Pasteurella multocida on blood agar.

Legionella pneumophila

Waterborne intracellular bacillus causing Legionnaires’ disease and Pontiac fever; requires L-cysteine and iron.

BCYE Agar

Buffered charcoal-yeast-extract agar supplying L-cysteine and iron for Legionella isolation.

L-Cysteine Requirement

Essential amino acid needed for Legionella growth, used diagnostically in specialized media.

Legionnaires’ Disease

Severe atypical pneumonia with fever, cough, possible multi-organ involvement caused by L. pneumophila.

Pontiac Fever

Mild, non-pneumonic, flu-like illness linked to Legionella exposure.

Ground-Glass Colonies

Blue-green, glistening colonies with finely granular center typical of Legionella pneumophila on BCYE.