First Aid Microbiology

Question 1

What is the treatment of choice for MRSA?

Answer 1

Nafcillin, a beta-lactamase resistant penicillin

Remember! Naf for Staph.

Question 2

Where do the Ig binding proteins we use in lab come from?

Answer 2

Protein A - Staphylococcus aureus

Protein G - Streptococcal species

Question 3

Alpha, beta, and gamma hemolysis

Answer 3

Alpha: Oxidation of hemoglobin to methaglobin, which looks greenish

Beta: Complete catabolism of heme (hence the halo)

Gamma: No hemolysis

Question 4

Most common cause of endocarditis of prosthetic heart valves

Answer 4

Staphylococcus epidermitis

The enemy of all prosthetics. It can stick to metal because of its sticky biofilm. This biofilm also reduces immune cell and antibiotics.

Treat w/ vancomycin or novobiocin.

Question 5

What is the significance of S. pyogenes protein M?

Answer 5

Protein M is a major virulence factor for S. pyogenes, and is the cause of rheumatic fever.

Protein M shares epitopes with cardiac myosin, resulting in the autoimmune features of rheumatic fever. This is a sequellae of strep pharyngitis, but NOT after strep skin infections, unlike PSGN which may occur after either.

Rheumatic fever is characterized by Jones criteria: Arthritis, acute heart symptoms, subcutaneous nodules, erythema marginatum, and Sydenham’s chorea.

Question 6

Role of antibiotics in rheumatic fever vs PSGN

Answer 6

Early penicillin treatment of Strep pharyngitis can prevent rheumatic fever. However, it does not reduce the risk of PSGN.

Question 7

Patient presents with bacterial skin infection. Cultures indicate that the causative organism is a bacitracin-sensitive streptococcus.

Which group of streptococci must this be?

Answer 7

Group A streptococci

aka Strep. pyogenes

All others are bacitracin resistant

Question 8

Streptolysin O titer

Answer 8

Titer against a strep enzyme antibody.

Useful for retroactive diagnosis of strep infection in post-infectious sequellae, such as rheumatic fever and PSGN.

Question 9

CAMP test

Answer 9

CAMP test + means Streptococcus agalacticae

Question 10

1 cause of meningitis in neonates

Answer 10

Group B streptococci

aka Streptococcus agalacticae

They acquire it from their mother as they exit the vaginal canal. So, at 35 weeks pregnancy, all pregnant women are swaved and tested for GBS. If it is there, the mother is given penicillin INTRAPARTUM (during delivery) to elimiate the bacteria.

Question 11

Differentiating the streptococci

Answer 11

Question 12

Hippurate hydrolysis

Answer 12

+ indicates GBS is present

Question 13

Streptococcus pneumoniae “MOPS” mnemonic

Answer 13

Meningitis

Otitis media

Pneumonia

Sinusitis

Question 14

Gram + cocci growing in chains are obtained from a patient’s cultures. A sample is swaved on a blood culture plate and the below shows the results.

What tests can you do to come to a final diagnosis?

Answer 14

The organism is a streptococcus that is alpha hemolytic. So, it must be either Streptococcus pneumoniae or one of the viridans group Streptococci.

To differentiate these, bile solubility and optochin sensitivity may be tested. S. pneumoniae is susceptible to both, while viridans streptococci are resistant to both.

Question 15

Viridans group streptococci are known to cause transient bacteremia following dental procedures. How is this possible and what are possible consequences? Can this be prevented?

Answer 15

Viridans group streptococci adhere to platelets via dextrans, enabling them to be swept up into the bloodstream. From here, they may colonize damaged heart valves by hopping onto platelet-fibrin aggregates at this location, causing endocarditis (most commonly the mitral valve, since mitral valve damage is highly prevalent)

This may be prevented with antibiotic prophylaxis prior to the procedure, often penicillin or a lincosamide.

Question 16

Differentiating E. faecium from E. faecalis

Answer 16

E. faecium is rarer, but much more pathogenic and causes more severe UTIs, cardiac infections, and biliary tree infections. E. faecium is also multidrug resistant, including resistant to vancomycin, and linezolid is first-line.

Both can grow in 6.5% NaCl and are bile-resistant. (They have to be bile-resistant since they are enterococci!) They are prominent causes of nosocomial infections.

Question 17

Gram + rods in chains with poly-D-glutamic acid capsule. . .

Answer 17

. . . indicates Anthrax

Treat w/ fluoroquinolones and doxycycline

Question 18

Anthrax toxins

Answer 18

• Edema factor: an adenylate cyclase that induces nonspecific inflammatory cytokine production, causing systemic edema.
• Lethal factor: Cleaves MAPK, causes necrosis (manifested as eschars)

Question 19

Patient presents with vomiting, diarrhea, and abdominal pain. Patient believes symptoms started after consuming leftover fried rice that had “gone bad”.

What is the likely diagnosis?

Answer 19

Bacillus cereus infection

Question 20

Mechanism of tetanus toxin

Answer 20

Protease that cleaves the SNARE proteins on inhibitory interneurons (called Renshaw neurons), preventing release of inhibitory glycine and GABA that normally help to reduce muscle tone.\

aka Tetanospasmin. Should be called TetanoSNAREase.

Question 21

Ascending paralysis vs descending paralysis

Answer 21

Descending is characteristic of botulism

Ascending is characteristic of Gullian-Barre

Question 22

Mechanism of botulinum toxin

Answer 22

Protease that cleaves the SNARE proteins on motor neurons, preventing release of acetylcholine, leading to flaccid paralysis.

Should be called BotulinoSNAREase.

Question 23

Ingestion of botulinoSNAREase in babies vs adults

Answer 23

• In adults, botulism is usually caused by ingestion of pre-formed toxin in poorly canned foods, without colonization.
• In babies, botulism is usually caused by colonization by spores found in honey or corn syrup.

Question 24

When do you give vancomycin orally?

Answer 24

Basically just for pseudomembranous colitis

It has poor oral absorption, but that’s a good thing if your target is in the GI tract!

The rest of the time you give it IV.

Question 25

Pediatric patient presents to clinic with infection of the oropharynx. Soon after interview begins patient begins wheezing. Cervical lymphadenopathy is noted on brief physical exam, as it absent gag reflex. Patient's family immigrated from El Salvador two years ago. Soon after workup is begun and patient is admitted, patient develops hypotension. ECG shows ventricular fibrillation. What other diseases is this individual at elevated risk for?

Answer 25

This presentation is consistent with Corynebacterium diptheriae. Immigration status and apparent sensitivity to diptheria toxin indicates that this individual likely never received the **TDaP vaccine.** Consequently, they are also at risk for **tetanus and pertussis.**

Question 26

Pediatric patient presents to clinic with infection of the oropharynx. Soon after interview begins patient begins wheezing. Cervical lymphadenopathy is noted on brief physical exam, as it absent gag reflex. Patient's mother reports that she has been hesitant to have the child vaccinated due to concerns over thalidomide. What is the likely diagnosis?

Answer 26

Corynebacterium diptheriae infection of the oropharynx. Pseudomembranes in the oropharynx that are not consistent with thrush should immediately make you think of diphtheria, especially in unvaccinated individuals. If the toxin makes it to the bloodstream, it can lead to life-threatening myocarditis with **arrhythmia** and heart block. **Albert's and Ponder's stain** may be used to demonstrate polar metachromatic granules. **Elek's test** may be performed to evaluate whether or not strain is toxin-producing (X marks the spot, presence of toxin). **Antitoxin should be administered immediately.**

Question 27

Clinically relevant gram + cocci

Answer 27

Staphylococci Streptococci Enterococci (which were formerly streptococci!)

Question 28

Clinically relevant gram + bacilli

Answer 28

**B**acilli (anthracis, cereus) **C**lostridia (tetani, botulinum, difficile, perfringens) **C**orynebacteria (diphtheria) **L**isteria (monocytogenes) **Remember, BCCL ≈ bacilli**

Question 29

Why shouldn't pregnant women eat soft cheeses or drink unpasteurized milk?

Answer 29

They may harbor ***Listeria monocytogenes***, which are very teratogenic and may terminate pregnancies.

Question 30

Classical empiric treatment for meningitis is ceftriaxone + vancomycin, to cover gram + and gram - . However, in infants, hose above age ~65, and immunocompromised individuals, ampicillin is also added. Why?

Answer 30

**To cover *Listeria monocytogenes*,** which is a common cause of meningitis in these populations, but not children/adolescents/younger adults (ie, those with immune systems that are in their prime. ) Remember, *Listeria* is an intracellular organism that knows how to work the macrophage. It takes some strong cell-mediated immunity to be able to clear it.

Question 31

Patient presents with new jaw swelling and raised erythematous lesion at the angle of the mandible. Patient has recent history of maxillofacial surgery. On exam, the abscess has draining sinus tracts under the skin. Aspirate is taken and cultured successfully, yielding branching, filamentous, and obligate anaerobic gram positive rods. Histology of the tissue reveals sulfur granules. What is the diagnosis?

Answer 31

**Cervicofacial actinomycosis**, caused by **Actinomyces israelii** Sulfur granules (shown below) and sinus tracts are pathognomonic in the context of facial abscess and recent dental/maxillofacial operation. **Treat w/ penicillin G and abscess drainage.**

Question 32

Patient with history of recent liver transplant, currently on immunosuppression, presents with dyspnea, chest pain, and cough productive of bloody, purulent sputum. Patient reports that they have been careful to stay home and minimize exposure to sick contacts and have instead been spending most of their time alone in their home garden. CXR shown below. Sputum culture reveals gram positive filamentous, branching rods that grow in aerobic conditions. What further infectious complications is this individual at risk for? What is the treatment of choice?

Answer 32

Patient is likely infected with Nocardia asteroides, and is at risk for encephalitis and brain abscesses. The lung CXR shows a cavitating abscess, characteristic of pulmonary nocardiasis. Patient should be treated with **sulfonamides.**

Question 33

Organisms that cause HUS following a prodromal diarrhea

Answer 33

* Shigella dysenteriae * EHEC

Question 34

"Traveller's diarrhea"

Answer 34

Aka ETEC Transmitted via water. Causes watery diarrhea, NOT dysentery. Caused by two toxins: Heat labile toxin -- increases cAMP Heat stable toxin -- increases cGMP "e**L** **A**gua de **S**an **G**abriel"

Question 35

15 month toddler presents with fever of 39^oC and loose, watery stools which developed over two days. Toddler had been previously healthy with uncomplicated pregnancy and delivery and meeting all developmental milestones. Parents report that they are confident the child did not ingest anything as they have been very careful to child-proof the home and watch over their child, who spends much of their time playing with toys or the family's pet dog. Tenderness in the right lower quadrant is detected on physical exam, but exam is otherwise within normal limits. An organism closely related to the likely cause of this toddler's disease has caused epidemic outbreaks through spread by which vector?

Answer 35

Fleas This child is likely suffering from Yersinia enterocolitica infection, as evidenced by watery diarrhea with abdominal pain mimicking appendicitis and close contact with dogs, the main hosts of Yersinia enterocolitica. Yersinia pestis causes bubonic plague, an epidemic disease that commonly uses rats as a host and spreads to humans through fleas as a vector. Praire dogs are also common carriers of the disease in the Americas.

Question 36

A 46-year-old Caucasian man, a physician from a town in central Serbia, had nausea, fever, and had suffered bloody diarrhea that lasted for one day. The diarrhea was self-limiting and he was treated only with antipyretic drugs. Ten days later, he felt muscle weakness in his lower extremities. Proximal weakness in his legs in difficulty climbing stairs and rising from a low chair. During the following days weakness also appeared in his arm muscles, and gradually increased in his upper and lower leg muscles. On neurologic exam, patient's mental status and cranial nerves were normal. CBC, muscle enzymes, and BMP returned normal. What was the most likely etiology of this individual's diarrhea?

Answer 36

***Campylobacter jejuni*** Campylobacter is one of the major causes of dysentery, and is often acquired from poultry (although it has reservoires in the guts of many organisms, so many types of meat can be contaminated, chicken is the most frequent). Camplyobacter is noted for its association with the development of Gullian-Barre syndrome, an autoimmune syndrome of ascending paralysis, through the development of autoantibodies against peripheral nerve gangliosides.

Question 37

Patient presents with bloody diarrhea. On CT, patient is found to have a right liver abscess. Histology from the patient is shown below. What is the proper treatment for this patient's condition?

Answer 37

This patient is presenting with **entamoeba histolytica enteritis.** Entamoeba histolytica is the only significant amoebal cause of dysentery (all the others are gram negative rods). It commonly causes **hepatic abscesses**, usually on the right, with an intraluminal consistency of **"anchovy paste".** Histology shows multiple entamoebae with phagocytosed red cells. **Metronidazole** is first-line. For pregnant women or following first-line treatment, **paromycin** is preferred. Iodoquinol may be used to treat intraluminal-limited disease, but is rarely used. **Note:****Even when there is an abscess, metronidazole is preferred to surgical drainage.**

Question 38

Mycobacterium avium complex

Answer 38

* "Complex" because two specices: avium and intracellulare * Organisms are ubiquitous in water or soil, so Hx may not be helpful apart from HIV or no * Very different presentations in healthy individuals and AIDS patients * **Healthy individuals:** * **Pneumonia w/ bronchiectasis, usually middle-aged** * Treat w/ **azithromycin, rifampin, ethambutol** * **Individuals w/ AIDS:** * Disseminated infection with **fever, weight loss, bone marrow suppression, lymphadenopathy, hepatosplenomegaly** * Treat w/ **macrolide plus ethambutol.** Add rifampin if patient not on consistent antiretroviral therapy.

Question 39

Tuberculoid vs lepromatous leprosy

Answer 39

Question 40

"Weakly acid fast staining" vs "Strongly acid fast staining"

Answer 40

"Weakly acid fast staining" = Nocardia asteroides "Strongly acid fast staining" = Mycobacteria