Question

41. Explain how a PPD works. Include why someone from another country would have a positive PPD test if they have never had the disease? How would they confirm or deny this?

42. Identify a disease that has a high incidence of asymptomatic carriers. Where does the organism hide in the body? How does this influence the spread of a disease?

43. Compare and contrast food poisoning by intoxication and infection by time of onset, treatment, and mode of action. Give one example of each type.

44. Describe the virulence factors of Staphylococcus aureus and how they increase virulence. Name at least 2 diseases that are caused by this organism.

Answer 1

41.

Tuberculosis (TB) is a leading cause of morbidity and mortality in developing countries. Tuberculin skin test (TST) is the only method of detecting M. tuberculosis infection in an individual. The tuberculin most widely used is purified protein derivative (PPD), which is derived from cultures of Mycobacterium tuberculosis, the causative organism of TB. A standard dose of five tuberculin units (TU) (0.1ml) is injected intradermally (into the skin) and read 48 to 72 h later. The reaction to intracutaneously injected tuberculin is the classic example of a delayed hypersensitivity reaction. Usually an induration of 10 mm or more is considered as positive.

PPD tests only the state of infection and not of disease. A person from a country with a high prevalence of the disease may have been infected but mounted a successful immune response to avoid an ongoing infection and subsequent disease. Thus the findings of a PPD test is difficult to interpret and a disease state has to be confirmed by other tests like sputum microscopy, culture, Quantiferon gold test, xrays etc.

42.

An asymptomatic carrier is a person or other organism that has become infected with a pathogen, but displays no signs or symptoms. Typhoid fever is a disease caused by the bacterium Salmonella enterica ser. It has a high incidence of asymptomatic carriers. Typhi which tends to reside in macrophages. Mary Mallon, known as "Typhoid Mary", infected about 50 people before she was quarantined.

43.

Foods contaminated with pathogenic microorganisms are of two types: infection and intoxication. Food poisoning by intoxication is due to preformed toxins liberated into the food by the microorganisms. Eg. Botulism. Food poisoning by infection is due to the microorganisms growing and establishing themselves in the human intestinal tract. Eg. Salmonella typhimurium

44.

The virulence factors of Staphylococcus aureus include antigens (capsule, adhesins), enzymes (coagulase, lipase, hyaluronidase, staphylokinase, nuclease) and toxins (α –Toxin, β –Toxin, δ-Toxin, P-V Leukocidin, enterotoxin, exfoliative toxin, Toxic Shock Syndrome Toxin). The virulence of S. aureus is multifactorial and due to the combined action of several virulence determinants. This is except toxic syndromes and foodpoisoning. Toxic Shock Syndrome Toxin as the name implies is implicated in Toxic Shock Syndrome. Staphylococcal enterotoxins are involved in food poisoning.

References:

http://www.idoj.in/article.asp?issn=2229-5178;year=2012;volume=3;issue=1;spage=2;epage=6;aulast=Nayak

https://en.wikipedia.org/wiki/Asymptomatic_carrier#Typhoid_fever

https://www.news-medical.net/health/Staphylococcus-Aureus-Virulence-Factors.aspx