Haemophilus and Other Fastidious Gram-Negative Rods

Comprehensive practice flashcards covering the laboratory identification, virulence, clinical infections, and diagnostic characteristics of Haemophilus and other fastidious Gram-negative rods.

Which bacterial genera belong to the fastidious group of Gram-negative bacilli outlined in the lecture?

Haemophilus, HACEK group, Legionella, Bordetella, Pasteurella, Brucella, and Francisella.

What is the typical microscopic morphology and Gram reaction of Haemophilus species?

Gram-negative pleomorphic coccobacilli or rods (seen more commonly in smears of colony growth).

What are the motility and atmospheric growth requirements of Haemophilus species?

They are nonmotile and aerobic or facultatively anaerobic.

What are the general oxidase and catalase reactions for the genus Haemophilus?

They are both oxidase positive and catalase positive.

What are the three major pathogenic species of Haemophilus?

Haemophilus influenzae, Haemophilus aegyptius, and Haemophilus ducreyi.

Why can sheep blood agar (SBA) not be used to grow Haemophilus species in pure culture, and what medium is used instead?

SBA does not supply available X and V factors needed for pure growth; chocolate agar is used because it provides both X and V factors.

What proportion of normal adult human respiratory flora consists of Haemophilus species?

Approximately $10\%$ of normal respiratory flora (including H. parainfluenzae and nonencapsulated H. influenzae).

What is satellitism in relation to Haemophilus species on sheep blood agar?

The growth of Haemophilus colonies strictly surrounding colonies of other organisms that supply the required V factor.

Which helper organisms can demonstrate satellitism with Haemophilus species on sheep blood agar (SBA)?

Staphylococcus aureus, Streptococcus pneumoniae, and Neisseria species.

What is the factor requirement rule for Haemophilus organisms containing 'para' in their species name?

Organisms with 'para' in their name require V factor only.

What is X-factor and what are its physical stability properties?

X-factor is hemin, which is a heat-stable substance required for hemoglobin synthesis.

What is V-factor and what are its physical properties and chemical identity?

V-factor is nicotinamide adenine dinucleotide (NAD, or coenzyme I), which is heat-labile and found in blood or secreted by certain organisms.

On what cellular structure is the serotyping of Haemophilus influenzae based?

The capsular polysaccharide.

How many distinct capsular serotypes are identified for Haemophilus influenzae?

Six serotypes, designated a, b, c, d, e, and f.

Which serotype of Haemophilus influenzae is recognized as the most invasive and most common in children?

Serotype b (Hib).

What severe clinical conditions does Haemophilus influenzae type b tend to cause in children under $18\text{ months}$ old?

Meningitis and septicemia.

What airway disease does Haemophilus influenzae type b typically cause in children older than $18\text{ months}$?

Epiglottitis (causing airway swelling and obstruction).

What respiratory tract illnesses can Haemophilus influenzae type b cause in general populations?

Pharyngitis and laryngotracheobronchitis (which often becomes croup).

What spectrum of diseases differentiates encapsulated from nonencapsulated Haemophilus influenzae strains?

Encapsulated strains cause invasive diseases (cellulitis, arthritis, epiglottitis, meningitis, bacteremia), whereas nonencapsulated strains generally cause localized infections.

Under what conditions do nonencapsulated Haemophilus influenzae strains cause invasive disease?

Meningitis in immunocompromised or chronically ill adults, or neonatal sepsis and lower respiratory tract infections in children.

What localized clinical infections are commonly caused by nontypable Haemophilus influenzae?

Otitis media, sinusitis, bronchitis, adult pneumonia, and urinary tract infections.

What viral illness was Haemophilus influenzae originally and mistakenly thought to cause?

Influenza ('the flu'), which was later discovered to be caused by a virus.

What disease is caused by Haemophilus ducreyi, and where is it most commonly encountered?

Chancroid (a sexually transmitted infection); it is not normal flora and is found mostly in developing countries.

What are the clinical characteristics of the genital lesion in chancroid caused by Haemophilus ducreyi?

A painful soft chancre consisting of genital ulcers with irregular edges.

What are buboes in the context of a Haemophilus ducreyi infection?

Suppurative, enlarged, inflamed, and draining inguinal lymph nodes that may form abscesses.

Which organism and characteristic Gram stain arrangement are shown in this microscopic image?

Haemophilus ducreyi, showing its characteristic 'school of fish' arrangement.

What common pediatric eye condition is caused by Haemophilus influenzae biogroup aegyptius?

Conjunctivitis (also known as 'pinkeye').

What severe systemic condition is caused by Haemophilus influenzae biogroup aegyptius in hot climates, and what is its potential mortality rate?

Brazilian purpuric fever (BPF), with mortality rates reaching up to $70\%$.

What are the clinical manifestations of Brazilian purpuric fever (BPF)?

Conjunctivitis, high fever, abdominal pain, vomiting, petechiae, purpura, septicemia, and shock.

What clinical specimen types are routinely submitted for the laboratory recovery of Haemophilus species?

Blood, CSF, middle ear exudates, joint fluids, upper and lower respiratory tract specimens, conjunctival swabs, vaginal swabs, and abscess drainage.

What laboratory processing and media are required for CSF specimens suspected of containing Haemophilus?

Direct microscopic examination (cytocentrifuge recommended for Gram stain) and plating to SBA, CHOC, and THIO.

What plating media are utilized when processing synovial fluid specimens for Haemophilus?

SBA, CHOC, THIO, and Thayer-Martin agar.

What plating media are utilized when processing lower bronchial specimens (brush, wash, lavage)?

SBA, CHOC, and MAC.

How does the microscopic morphology of Haemophilus species vary between clinical specimens and pure culture growth?

In clinical specimens, they appear as small regular coccobacilli; in pure culture, they more commonly appear as long filaments.

Describe the colonial morphology and odor of Haemophilus influenzae on chocolate agar.

Translucent, moist, smooth, convex, grayish colonies with a distinct 'mousy' or 'bleach-like' odour.

What are the growth factor requirements of Haemophilus influenzae?

It requires both X factor and V factor ($\text{X}^+, \text{V}^+$).

Based on this growth factor test showing growth around the V strip only, which organism is identified?

Haemophilus parainfluenzae (requires V factor only).

What are the X and V factor requirements of Haemophilus ducreyi?

It requires X factor only ($\text{X}^+, \text{V}^-$).

What is the clinical purpose of performing a porphyrin test on Haemophilus isolates?

To differentiate heme-producing strains from those requiring exogenous X factor by evaluating synthesis of porphyrins from delta-aminolevulinic acid (ALA).

In this porphyrin test viewed under ultraviolet light, which organism requires exogenous X factor?

The top organism, which shows a negative porphyrin reaction (non-fluorescent, indicating lack of heme synthesis enzymes and a requirement for X factor).

Which Haemophilus species display beta-hemolysis on rabbit blood agar?

Haemophilus haemolyticus, Haemophilus parahaemolyticus, and Haemophilus parainfluenzae (biotypes 2 and 3).

What are the growth factor requirements of Haemophilus aphrophilus?

It requires neither X factor nor V factor ($\text{X}^-, \text{V}^-$).

Which bacterial genera make up the HACEK acronym?

Haemophilus (Aggregatibacter), Actinobacillus, Cardiobacterium, Eikenella, and Kingella.

What are the common general physiological characteristics of the HACEK group?

They are slow-growing Gram-negative bacilli, capnophilic (require $5\%-10\%\text{ }\text{CO}_2$), fail to grow on MacConkey agar (MAC), and are normal flora of the oral cavity.

What major cardiovascular infection is significantly caused by HACEK organisms?

Endocarditis (specifically subacute bacterial endocarditis).

What were the former taxonomic designations of Aggregatibacter aphrophilus?

Hemophilus aphrophilus and Hemophilus paraphrophilus.

What are the catalase, oxidase, and indole reactions for Aggregatibacter aphrophilus?

Oxidase variable, catalase negative, and indole negative.

What clinical scenarios and wound types are typically associated with Eikenella corrodens infections?

Mixed infections resulting from human bites or clenched-fist wounds.

Describe the unique colonial characteristics and odor of Eikenella corrodens after $48\text{ hours}$ of incubation.

Small colonies with clear centers surrounded by flat spreading growth that pits/corrodes the agar, produces a yellowish hue, and emits a bleach odor.

What specialized medium is shown here for the isolation of Legionella pneumophila, and how is the pathogen acquired?

Buffered charcoal yeast extract (BCYE) agar; the infection is acquired primarily through inhalation of aerosols.