A 3-month-old boy is brought to the emergency department due to 2 days of fever and irritation that progressed to lethargy. Temperature is 38.6 C (101.5 F). A lumbar puncture is performed, and the results of cerebrospinal fluid (CSF) analysis are as follows:
Glucose 30 mg/dL Protein 180 mg/dL Leukocytes 1,000/mm3
Gram-negative coccobacilli are isolated from the CSF. These organisms demonstrate little growth on 5% sheep blood agar but grow well when incubated on the same media alongside colonies of Staphylococcus aureus. Staphylococci promote the growth of these bacteria by supplementing which of the following substances?
This patient likely has Haemophilus influenzae type b meningitis (which may occur in patients who are unvaccinated or immunocompromised). Haemophilus (from the Greek for blood-loving) requires both X factor (hematin) and V factor (NAD+) to support growth. Sheep blood agar does not allow Haemophilus growth due to insufficient nutrients and the V factor-inactivating enzymes in the medium.
Growth of Haemophilus species on sheep blood agar can be achieved by cross-streaking the medium with Staphylococcus aureus. H influenzae colonies will grow around the streaks of β-hemolytic S aureus colonies because the latter actively secrete V factor (NAD+) into the medium and facilitate release of additional X factor (hematin) from β-hemolysis-induced erythrocyte lysis. This is known as the "satellite" phenomenon.
(Choices A and F) Pyruvate is the end product of glycolysis and is a starting substrate for the synthesis of glucose, lactate, fatty acids, amino acids, and nucleic acids. Bacteria use pyruvate in the metabolism of sugars for energy in the form of ATP.
(Choice B) Catalase is an enzyme present in staphylococci; the presence or absence of this enzyme is used to differentiate staphylococci from streptococci.
(Choice C) Iron is an essential growth factor for many bacteria, and it is provided in all blood agars from the iron within erythrocytes.
(Choice D) The use of lactose fermentation as a source of energy is best known in Escherichia coli with the well studied lac operon, an inducible and repressible genetic sequence coding for the enzymes necessary for lactose fermentation in the absence of glucose. It is activated by a sensed glucose deficit and repressed when glucose is available again.
Educational objective:
Haemophilus influenzae is a "blood-loving" organism that requires X (hematin) and V (NAD+) factors for growth. This can be accomplished by growing H influenzae in the presence of Staphylococcus aureus and demonstrating the "satellite phenomenon," whereby H influenzae grow only near the β-hemolytic S aureus colonies that produce the needed X and V factors.