Haemophilus influenzae are Gram-negative diplococci that actually have many different strains and are part of the normal flora of the nasopharynx. There’s one strain that causes the most pathology in humans which is Type B.
H. influenzae Type B (HiB) is encapsulated and has a polyribosyl ribitol phosphate (PRP) capsule which is it’s main virulence factor. Since it’s the capsule that is so virulent patients who are more susceptible to encapsulated bacterial infections are more at risk to this organism. That includes patients who have Sickle Cell Disease and patients without spleens.
When you think about the diseases that Haemophilus influenzae causes, just think about all the basic respiratory illnesses. It is one of the top three causes of bacterial acute rhinosinusitis. In children it can also cause epiglottitis which presents with dyspnea, fever, difficulty swallowing, drooling, hoarseness, and erythemat that is often described as cherry-red. It can swell the airway shut, so this is a medical emergency. Haemophilus influenzae can also be a cause of meningitis in patients under the age of 18. It can also cause pneumonia, otitis media, and exacerbation of COPD.
Haemophilus spp. can only grow on Chocolate agar because the bacterium requires Factors V and X, which exist inside red blood cells. However, it doesn’t have any hemolysins to lyse those red blood cells, so it won’t grow on any blood agars. It will only grow on Chocolate agar which is an agar of lysed red blood cells.
There is a conjugate vaccine that contains the PRP capsular antigen conjugated to either an antigen from Neisseria meningitidis or the Tetanus toxin.
You can treat Haemophilus influenzae infections with ceftriaxone and give any close contacts to an infected individual rifampin to prevent them from developing disease.
Haemophilus ducreyi shares a genus but is a TOOOOTALLY different clinical manifestation. Best to not mix these up.
H. ducreyi does share a lot of characteristics with Haemophilus influenzae like the fact that it’s a Gram-negative, encapsulated coccus, that more often affects patients without spleens and patients with Sickle Cell Disease.
Now for the differences. Haemophilus ducreyi is more often found in the tropics and is transmitted through sexual contact without barrier methods. It will cause painful, genital ulceration that can be a single lesion or multiple. This is called a chancroid. This is often accompanying enlarged, tender unilateral inguinal lymph nodes that swell so much that they ulcerate. Once diagnosed, this can be treated with azithromycin.
A 14-year-old male presents to the emergency department with a reduced level of consciousness for the past 5 hours. The previous day he had a headache and vomiting. Vital signs were normal. Physical examination revealed a Glasgow Coma Score of 9, a positive Kernig test, but no focal neurological deficits. There was not a petechial rash. Analysis of the cerebrospinal fluid revealed an elevated white blood cell count and protein levels, and a decreased glucose level compared to the level in the serum. Serum analysis revealed neutrophilia and a high C-reactive protein level. The cerebrospinal fluid was cultured on blood agars, but produced no growth. The fluid cultured on chocolate agars produced small white colonies of a Gram-negative organism. What is the most likely causative agent?
A. Neisseria meningitidis
B. Listeria monocytogenes
C. Streptococcus pneumoniae
D. Haemophilus influenzae
First, diagnose the patient.
This patient is presenting with headache, vomiting, reduced consciousness, and neck stiffness (positive Kernig sign). Which suggests some sort of meningitis. The lumbar puncture revealed high WBC numbers, low glucose, and high protein, which suggests either a fungal or bacterial meningitis. Analysis of the serum revealed a predominance of neutrophils, which suggests a bacterial infection. Therefore this patient most likely has bacterial meningitis.
A. Neisseria meningitidis is a common cause of bacterial meningitis. It is also a Gram-negative organism, this organism would also only grow on Chocolate agars, except that this organism typically presents as meningitis with a petechial rash, which this patient didn’t have.
B. Listeria monocytogenes is a common cause of meningitis, but usually only affects the elderly. It is also a Gram-positive organism.
C. Streptococcus pneumoniae is a common cause of meningitis, but this organism would be able to grow on blood agars, and chocolate agars, and is a Gram-positive organism.
D. Haemophilus influenzae is a common cause of meningitis, it is Gram-negative, and it cannot grow on blood agar.
Therefore,
A 17-year-old male presents with burning while urinating and fever for the past 8 days. Vitals signs are normal. Physical examination reveals a single ulceration on the shaft of the penis. Laboratory studies reveal a negative Venereal Disease Research Laboratory Test. Gram staining revealed the presence of extracellular Gram-negative coccobacilli. What is the most likely causative agent?
A. Treponema pallidum
B. Klebsiella granulomatis
C. Herpes Simplex Virus
D. Haemophilus ducreyi
First, diagnose the patient.
This patient is presenting dysuria and fever which is highly suggestive of a urinary tract infection, but the physical examination revealed the genital ulceration, which suggests that something else is happening with this patient. Therefore, this patient has a genital lesion.
A. Treponema pallidum causes syphilis. Primary syphilis will present as a genital ulceration, but this will not be painful, even though it really looks like it should be. This patient also had a negative VDLR test, which is a non-specific screening test for syphilis. It is also a hard organism to Gram stain and is not coccobacillus, but a spirochete. Therefore, it is highly unlikely that this patient has syphilis.
B. Klebsiella granulomatis causes Donovanosis which can present as ulcerations, but they are typically surrounding the inguinal lymph nodes. This is also an obligate intracellular organism (think of those Donovan bodies inside macrophages).
C. Herpes Simplex Virus can form genital lesions, but this is a crop of vesicles that can crust over. This can be itchy, painful, or burning. This does form ulcerations. This is also a viral infection, and would therefore not produce any organisms on Gram-staining.
D. Haemophilus ducreyi causes chancroids which are painful ulcerations that can be single or multiple. They are Gram-negative coccobaccili that arrange in stacked lines that’s described as a “school of fish” when they are extracellular.
Therefore,
References:
https://www.jstage.jst.go.jp/article/internalmedicine/46/4/46_4_195/_pdf/-char/en
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3669774/
https://microbewiki.kenyon.edu/index.php/Haemophilus_ducreyi
https://microbewiki.kenyon.edu/index.php/Haemophilus_influenzae
http://www.indianjournals.com/ijor.aspx?target=ijor:ijphrd&volume=5&issue=1&article=044
The Bradley Lab 