Question

1. All of the following are correct about Staphylococcus aureus, EXCEPT:

most strains are resistant to penicillin and ampicillin

it causes furuncles, carbuncles, and abscesses.

it is the coagulase-positive species

it is a common cause of strep throat

2. Acanthamoeba is NOT

a cause of keratitis

entering typically through broken skin or the conjunctiva

acquired by swimming in contaminated water

a heleminth

3. Fifth disease is normally

treated intensively with intravenous antibiotics

treated with topical antibiotic ointments

prevented through vaccination

allowed to resolve without treatment

4. The skin is an effective barrier against many pathogens. All the following describe ways that pathogens penetrate the skin EXCEPT for which one?

Helminths may burrow into the skin, creating their own paths to deeper tissues

Microbes may enter the body through skin that has been damaged, allowing them to reach deeper tissues

The loa loa worm enters the bloodstream through the bite of the deerflies that carry it

Bacteria on the surface of the skin often gradually consume the epidermis and dermis, creating a pathway to deeper parts of the body

5. All of the following are correct about Streptococcus pyogenes, EXCEPT

it causes scalded skin syndrome

humans are the only significant reservoir

some of the streptococcal toxins act as superantigens

infection is generally through direct contact

Answer 1

1) Staphylococcus aureus is a Gram-positive, round shaped bacterium that is positive for catalase, nitrate reduction and coagulase. It is a facultative anaerobe and can cause a wide range of illness from minor skin infections, such as pimples, impetigo, furuncles, carbuncles, cellulitis, scalded skin syndrome and abscesses to life threatening diseases, such as pneumonia, meningitis, endocarditis, osteomyelitis, etc. Most strains of Staphylococcus aureus are resistant to antibiotics like penicilin and ampicilin. The emergence of methicillin resistant S.aureus (MRSA) is a major problem to treat diseases caused by this bacterium. Staphylococci can be differentiated from Streptococci by the catalase test. Streptococci pyogenes is the causative agent of 'Strep throat' (Streptococcal Pharyngitis), which is characterized by acute rheumatic fever and post streptococcal Glomerulonephritis. So option (d) is the correct answer.

2) Acanthamoeba are protozoa that can be found in soil, air, sewage, seawater, contaminated swimming pools, domestic tap water, hospitals and contact lens cases. The mode of transmission includes inhalation of cysts and trophozoites from faeco-oral route, improper contact lens care practices or direct skin contact by traumatic injections, or through broken skin or conjunctiva. Acanthamoeba is responsible for diseases such as Keratitis and granulomatous amoebic encephalitis (GAE). The acanthamoebic keratitis can lead to corneal ulcers or even blindness. So from the given options, Acanthamoeba is not a helminth. Option (d) is the correct answer.

3) Fifth disease is also known as Erythema infectiosum, caused by Human parvovirus or Human B19 virus. The virus is linear, single stranded DNA virus characterized by rashes accompained with low grade fever, cold like symptoms. The facial rash looks as if the cheeks were slapped. There is generally no treatment available for this disease. There are no antivirals but IgG can be administered to immunocompromised patients with persistent infections. There is no vaccine available as well. An antihistamine drug can be used for itching of rashes and for those with joint pain, analgesics can be used. The virus clears up on its own, without treatment. So option (d) is the correct answer.

4) The skin is an effective barrier against many pathogens, but still several pathogens find its way to penetrate the skin. As for example, many parasitic helminths and protozoans use the skin or eyes as a point of entry. Some may physically burrow into the skin or the mucosa of the eye and others may breach the skin barrier by means of an insect bite. The loa loa worm enters into the blood stream through the bite of the deerflies that carry it. MIcrobes like bacteria may enter the body through skin that has been damaged, allowing them to reach deeper tissues. Bacteria initially low in number, colonize different layers of skin (epidermis, dermis, subcutaneous and adipose tissues and muscle fascia) and finally enter through wounds or bites that breach the skin barrier, creating a pathway to deeper parts of the body. So the false statement is 'bacteria on the surface of the skin often gradually consume the epidermis and dermis, creating a pathway to deeper parts of the body'. Option (d) is the correct answer.

5) Sterptococcus pyogenes is a gram positive bacterium responsible for diseases like scarlet fever, necrotizing fascitis, toxic shock syndrome, etc. They release erythrogenic toxin which are exotoxins in nature secreted by strains of Streptococcus pyogenes. Some of the streptococcal toxins act as superantigens. SpeA and SpeC are superantigens, which induce inflammations by non-specifically activating T cells and stimulating the production of cytokines. In Streptococcus infection, humans are the only significant reservoir and it mainly spreads through direct skin contacts in areas with poor hygiene and crowded living conditions. But Streptococcus pyogenes does not cause scalded skin syndrome, it is caused by Staphylococcus aureus, which is characterized by blisters and desquamation of skin, caused by exfoliative toxin of Staphylococcus aureus. So option (a) is the correct answer.