Microbiology Lecture: The Genus Haemophilus and Fastidious Gram-Negative Rods

Flashcards covering the characteristics, clinical significance, laboratory culture, factor requirements, and susceptibility testing of the genus Haemophilus.

What are the general Gram stain microscopic morphology and atmospheric requirements of Haemophilus species?

They are pleomorphic Gram-negative coccobacilli (which can also be filamentous), nonmotile, and aerobic or facultatively anaerobic.

Why are Haemophilus species unable to grow adequately on standard 5% sheep blood agar (SBA)?

Sheep blood agar contains unlysed red blood cells where essential X and V growth factors are not readily available; chocolate agar provides these released factors because the red blood cells have been lysed.

What are the specific chemical identities of the X and V factors required by Haemophilus species?

X factor is heme or hemin, and V factor is nicotinamide adenine dinucleotide (NAD).

What is satellitism as displayed by Haemophilus species on sheep blood agar?

The growth of tiny Haemophilus colonies clustered around colonies of organisms like Staphylococcus aureus, which secrete V factor (NAD) and/or lyse red blood cells to release growth factors.

Why must wooden applicator sticks rather than nichrome wire loops be used when performing the oxidase test?

Nichrome contains iron, which can reduce the oxidase reagent and cause a false-positive purple reaction.

Where are most Haemophilus species typically found as part of the human normal flora?

Mucous membranes, particularly in the oral cavity and upper respiratory tract.

Which encapsulated serotype of Haemophilus influenzae has traditionally been the most virulent pathogen, particularly in children?

Serotype b (Hib).

What life-threatening condition involving airway obstruction is classically caused by Haemophilus influenzae?

Epiglottitis (severe inflammation and swelling of the epiglottis).

What types of infections are typically caused by nonencapsulated (nontypable) strains of Haemophilus influenzae?

Localized infections such as otitis media, sinusitis, bronchitis, pneumonia, and conjunctivitis.

Which Haemophilus species is not considered normal flora and causes a sexually transmitted infection?

Haemophilus ducreyi.

What clinical disease and lesion type are caused by Haemophilus ducreyi?

Chancroid, which presents as non-suppurative, ulcerative lesions (soft chancres) on the genitals and inflamed lymph nodes (buboes).

What characteristic microscopic arrangement is observed on a Gram stain of Haemophilus ducreyi?

A "school of fish" arrangement.

What severe condition is caused by Haemophilus influenzae biogroup aegyptius, primarily observed in children in Brazil?

Brazilian purpuric fever (a severe systemic infection causing purulent conjunctivitis, sepsis, and septic shock with high mortality).

What is the primary diagnostic purpose of inoculating thioglycolate broth for sterile body fluids such as CSF or synovial fluid?

To ensure that low numbers of microorganisms that might fail to grow on solid agar plates are not missed.

Why can chocolate agar not be used as the base growth medium when performing X and V factor disk testing?

Chocolate agar already inherently contains both X and V factors, which would allow growth everywhere rather than restricting growth to the diffusion zones around the disks.

What growth factor requirement rule applies to Haemophilus species containing the prefix "para-" in their name (e.g., H. parainfluenzae)?

They require only V factor for growth.

On what specific types of media must hemolysis be evaluated for Haemophilus species?

Rabbit blood agar or horse blood agar (not sheep blood agar).

What does a positive ALA (porphyrin) test indicate about a Haemophilus strain?

It indicates that the organism is heme-producing (can synthesize its own X factor) and therefore does not require external X factor for growth.

What color change indicates a positive result in a nitrocephin disk test for beta-lactamase?

A color change to red or rusty red.

What is the clinical significance of a positive beta-lactamase test result on a Haemophilus isolate?

It indicates resistance to standard penicillins (ampicillin and amoxicillin), requiring treatment with extended-spectrum beta-lactams or cephalosporins.