Microbiology I Step I Review. Devin Gillaspie. First the Basics…. Gram Positive Organisms Stain purple/blue Cell wall + cell membrane Contains techoic acid – induces TNF and IL-1 Gram Negative Organisms Stain red Outer membrane + Periplasmic space + cytoplasmic membrane
A 60 year-old woman comes to the physician because of fevers, chills, and a cough for the past 2 weeks. She lives at home with her husband, has no chronic medical conditions, and has not travelled recently. Her temperature is 38.2° C (100.8° F). Physical examination reveals altered breath sounds. Chest X-Ray is abnormal. Which of the following is the most likely causal organism?
A. Chlamydia pneumoniae
D. Staphylococcus aureus
Correct answer is E. The most common cause of community acquired pneumonia is Sterptococcuspneumoniae. Choices A, B, and C are important causes of community-acquired pneumonia but are NOT the most frequent cause. Choice D (Staph aureus) is an important cause of CAP particularly in elderly patients with comorbidities, but is not the most common cause
Take home point: COMMON IS COMMON!!!
A three-week-old boy develops seizures, lethargy, and vomiting. Examination shows, a bulging fontanelle, and nuchal rigidity. Which of the following organisms should be suspected?
A. E. coli and Streptococcus agalactiae
B. H. influenza and Neisseriameningitidis
C. H. influenza and Streptococcus pneumoniae
D. Listeriamonocytogenes and Neisseriameningitidis
E. Staphylococcus aureus and Staphylococcus epidermidis
Correct answer is A. Microbial causes of meningitis varies by patient’s age and clinical setting. The most common causes of neonatal meningitis are Streptococcus agalactiae(Group B Strep), and E. coli.
H. Influenza was the most common cause of menigitis in ages 1-5 before 1990, but is much less common today after the introduction of an effective vaccine.
Neisseriamenigitidis is the most common in ages 5-40 (35% of children 5-15 yo, and 20% adults)
Streptococcus pneumoniae occurs in all ages (40% of adult cases, 15% of childhood cases) but is uncommon in neonates
Listeriamonocytogenes accounts for only 1% of bacterial menigits; seen in infants, elderly, and immunosuppressed.
Staphylococcus aureus is seen in post-surgical and post-traumatic meningitis
Staphylococcus epidermidis accounts for 75% of shunt-related meningitis
A 47 year-old man with a history of sickle cell disease has had numerous hospitalizations requiring the placement of IV lines. The patient has poor peripheral venous access, and a catheter is placed in the right subclavian vein. The patient subsequently develops right arm discomfort and swelling, and a temperature of 40.1°C with chills. Multiple blood cultures were taken and gram-positive cocci are isolated. The organism is catalase positive, and produces whitish colonies on mannitol salt agar. The colonies are gamma-hemolytic on blood sheep agar. Which of the following organisms is the most likely cause of this patient’s symptoms?
B. Staphylococcus aureus
C. Staphylococcus epidermidis
D. Streptococcus agalactiae
E. Streptococcus pyogenes
Correct answer is C. The patient has developed bacteremia. The description of the causative agent is consistent with a staphylococcal organism (catalase +, gram + cocci that grow on mannitol salt agar). The organism is therefore most likely Staphylococcus epidermidis because it was able to ferment mannitol and was not hemolytic (these characteristics rule out S. aureus).
Enteroccoccus and Streptococcus organisms are catalase negative. Both Strep species are Beta hemolytic and would not grow on mannitol salt agar, enterococcus is gamma hemolytic.
A 58 year old alcoholic male with multiple dental carries develops a pulmonary abscess and is treated with antibiotics. Several days later, he develops nausea, vomiting, abdominal pain, and voluminous green diarrhea. Which of the following antibiotics is most likely responsible for this patient’s symptoms?
Correct answer is B. Any time you see the development of diarrhea in the same question stem with “treated with antibiotics,” you should immediately think of pseudomembranous colitis. This is caused by Clostridium difficile and typically occurs as a result of treatment with clindamycin or ampicillin. You would confirm you suspicion by sending stool sample to be tested for C. diff toxin.
Chloramphenicol is not frequently used in the US and the most common side effect is aplastic anemia.
The key side effects of gentamycin are ototoxicity and nephrotoxiciy
Metronidazole and vancomycin are used to treat pseudomembranous colitis
A 16 year-old boy with sickle cell disease is hospitalized for a severe infection. His spleen has autosplenectomized, and he has had other minor infections in the past. His symptoms include fever, chills, cough, and chest pain. Bacteria from the patient’s sputum are optochin-sensitive organisms with a positive Quellung reaction. Which of the following is the most likely pathogen?
A. E. coli
B. H. influenza
C. Klebsiella pneumonia
E. Streptococcus pneumoniae
Correct answer is E. The combination of optochin sensitivity and positive Quellung reaction is characteristic of a single organism, Sterptococcuspneumoniae.
H. influenza, Klebsiella, and N. meningitidisare all encapsulated organisms and have positive Quellung tests, but none are optochin sensitive.