Fastidious Gram-Negative Bacilli and Sputum Culture Workup

Practice flashcards covering the HACEK group organisms (focusing on Aggregatibacter aphrophilus and Eikenella corrodens), key biochemical principles, and sputum specimen culture workup guidelines.

Under what genus was Aggregatibacter aphrophilus previously classified?

It was previously classified under the genus Haemophilus.

What atmospheric conditions are required for the growth of organisms in the HACEK group?

They are capnophilic and require $5\text{ to }10\text{ percent }CO_2$.

What is the primary, significant systemic infection associated with the HACEK group of organisms?

Subacute bacterial endocarditis.

Where are HACEK organisms typically found as normal human flora?

In the oral cavity and the upper respiratory tract.

Why do organisms in the HACEK group fail to grow on MacConkey agar?

They fail to grow on MacConkey agar because they are fastidious organisms.

Approximately how long does it take for HACEK organisms to produce significant colony growth on blood or chocolate agar?

They are slow-growing and take upwards of $48\,\text{hours}$ to produce significant colonies.

What are the microscopic morphological characteristics of Eikenella corrodens?

Long and thin gram-negative bacilli with rounded ends.

What distinctive colonial morphology features can be observed in extended incubations of Eikenella corrodens?

Yellowish pigmentation, a distinct center with a thin apron of growth (fried egg appearance), and corroding or pitting into the agar.

What specific type of wound or bite infection is classically associated with Eikenella corrodens?

Wound infections due to human bites, particularly clenched fist wounds resulting from fistfights.

What does the term asaccharolytic mean regarding Eikenella corrodens?

It means the organism does not ferment any sugars.

What is the oxidase reaction of Eikenella corrodens?

Oxidase positive.

What biochemical events indicate a positive nitrate reduction test?

The reduction of nitrate to nitrite, or further reduction all the way to nitrogen gas ($N_2$).

What organism serves as the classic quality control positive organism for the spot indole test?

Escherichia coli.

What are the substrate and enzyme involved in the indole biochemical test?

The substrate is the amino acid tryptophan, which is hydrolyzed by the enzyme tryptophanase to yield indole and ammonia.

According to the Mount Sinai criteria discussed, what threshold of squamous epithelial cells defines an acceptable quality sputum specimen?

Fewer than $25$ squamous epithelial cells per low-power field ($< 25\,\text{SEC/LPF}$).

In the lecture sputum case study, what organism was suspected based on heavy growth ($4+$) of pleomorphic gram-negative coccobacilli on chocolate agar?

Haemophilus influenzae.

Under the Mount Sinai laboratory standard operating procedure, when are probable respiratory pathogens identified from a sputum culture?

When they are present in moderate ($3+$) to heavy ($4+$) growth after $24\,\text{hours}$ to $48\,\text{hours}$ of incubation.

How can Streptococcus pneumoniae be differentiated morphologically from viridans group streptococci?

Streptococcus pneumoniae presents as lancet-shaped gram-positive diplococci with mucoid or flat/depressed (copper penny) colonies and strong alpha-hemolysis, whereas viridans streptococci form cocci in chains and pairs with smaller, semi-opaque, gray, drier colonies.