Introduction to medical microbiology

Question 1

What are possible infecting agents

Answer 1

• bacteria ,viruses ,fungi ,prions ,parasites

Question 2

What are some sterile sites found in the body?

Answer 2

-blood, CSF, lungs and bladder

Question 3

What are some non-sterile sites found in the body?

Answer 3

-nasopharynx, gut,, skin, urethra

Question 4

What does an organism found in a sterile area indicate?

Answer 4

It means the organism is related to the organism causing the disease/infection since sterile sites don’t contain pathogens naturally

Question 5

What are the specimen collections for bacterial culture for: UTI, chest infection, tonsillitis/ pharyngitis, wound or site infection, diarrhoea, bacteraemia, and meningitis

Answer 5

UTI - mid-stream urine 
chest infection = sputum (saliva+mucus) 
tonsilitis/ pharyngitis = throat swab 
wound or site of infection= swab or pus 
diarrhoea- faeces 
bacteraemia - blood 
meningitis - CSF (cerebro spinal fluid)

Question 6

What is unstained microscopy used for? (2)

Answer 6

1. to see pus in urine or CSF

2. to see parasites in faeces

Question 7

What is gram stain used for?

Answer 7

To see bacteria and yeast/fungi

Question 8

What is ZN stain (or auramine stain) used for?

Answer 8

To see mycobacteria e.g. TB

Question 9

What are main properties of gram stain?

Answer 9

• Very rapid diagnosis due to bacteria shape (telephone urgent information)
• Not sensitive
• Can’t usually identify particular species

Question 10

What are the main properties of bacterial culture?

Answer 10

• slow (can take days)
• sensitive (large volumes can be used)
• culture conditions need to be suitable for expected species (type of media (non-selective, selective), atmosphere, temperature and duration of incubation

Question 11

What is an example of a non-selective media?

Answer 11

• blood chocolate for specialised mycobacteria

Question 12

What is an example of a selective media?

Answer 12

MacConkey:nutrient agar with bile salts which supress the growth of gut bacteria but allows growth of bacteria such as e.coli (diarrhoea patients use this to suppress the growth of gut bacteria in natural body flora to identify harmful organism)

Question 13

What is alphahemolysis?

Answer 13

darker in colour, streptococcus pneumonia displays this, hydrogen peroxide in the bacteria oxidises the haemoglobin to green methemoglobin (breaks down RBCs to classify organism)

Question 14

What is betahemolysis?

Answer 14

lighter in colour, Streptolysin, an exotoxin, is the enzyme produced by the bacteria which causes the complete lysis of red blood cells

Question 15

What are some diagnostic methods for viruses? (5)

Answer 15

1. serology (study of blood serum for antibodies/immunity)
2. antigen detection
3. nucleic acid detection (sensitive)
4. more rare methods e.g. electron microscopy or tissue culture
5. molecular methods e.g. PCR (amplification of DNA)

Question 16

What are the 3 classification of parasites?

Answer 16

1. protozoa (malaria, amoebae, flagellates)
2. Helminths/ worms (flatworm, tapeworm, flukes)
3. Arthropods (lice, mites, ticks)

Question 17

What are the main diagnostic principles of parasitology? (2)

Answer 17

• microscopy of different stages (parasites, cysts and ova (POC) in faeces and blow films for malaria)
  -serology sometimes useful
  (culture difficult as complex life cycle)

Question 18

What are some examples of hospital acquired infections? (4)

Answer 18

• noravirus
• clostridium difficile
• MRSA (methicillin resistant staph aureus), +ve
• ESBLs:Organisms with Extended Spectrum β-lactamases (include variants of e.coli)