Classification of Pathogenic Bacteria

Question 1

Gram positive bacteria

Answer 1

Staphylococcus Aureus, Streptococcus Pneumoniae, Streptococcus Pyogenes, Clostridium Difficile

Question 2

Aerobic gram positive cocci

Answer 2

Staphylococci appear in clusters seen on a coagulase test. Streptococci and enterococci appear in chains and seen on agar plates. Identification of both is through morphology and physiology

Question 3

Staphylococcus aureus (gram positive)

Answer 3

Appearance; clusters, round, purple, coagulase positive. It is a commensal organism carried in the nose, axilla and perineum. Disease; boils/abscesses, soft tissue infection, septicaemia and osteomyelitis. Methicillin resistant staph aureus (MRSA) poses major problems for infection prevention and control in hospitals.

Question 4

Streptococcus pneumoniae (gram positive)

Answer 4

It is a-haemolytic so turns blood agar green. It causes pneumonia, meningitis and septicaemia.

Question 5

Streptococcus pyogenes (gram positive)

Answer 5

A major pathogen causing pharyngitis, cellulitis and necrotising fasciitis.

Question 6

Clostridium difficile (gram positive)

Answer 6

Bacilli, difficult to culture. Asymptomatic gut carriage in healthy people, But can cause diarrhoea, toxin production, pseudomembranous colitis and fatalities. It’s risk is increased with abx use. spreads via spores and is detected in a stool sample (antigen and toxin)

Question 7

Neisseria meningitidis (gram negative)

Answer 7

Cocci. Causes meningitis and septicaemia which is life threatening. Identified using PCR. Vaccine available

Question 8

Neisseria Gonorrhoea (gram negative)

Answer 8

Cocci. Causes urethritis in men and pelvic inflammatory disease in women. It is spread via sexual content.

Question 9

Escherichia coli. E-coli. (gram negative)

Answer 9

Bacilli. Found in human and animal reservoirs. It has several virulence mechanisms; pili/fimbriae, capsule and endo/exotoxins. It ferments lactose. Can cause UTIs. There are 160+ serotypes and strains vary considerably in disease potential e.g. enterotoxogenic E.Coli; travellers diarrhoea or enterohaemorrhagic E.Coli; bloody diarhhoea.

Question 10

Gram negative bacilli

Answer 10

Mainly coliforms a family of enterobacteriaceae. They are large intestine commensals. Lactose fermentation is a good preliminary test.

Question 11

Salmonella Spp (gram negative)

Answer 11

Bacilli. s.enteric has 1500 serotypes. It causes self-limiting enterocolitis with/without bloody diarhhoea. It is the second commonest cause of bacterial diarrhoea in the UK. It can be invasive and enter the blood steam.

Question 12

Helicobacter Pylori (gram negative)

Answer 12

Human stomach commensal which damages mucosa and causes ulcers. Risk factor; gastric adenocarcinoma.

Question 13

Haemophilus Influenza (gram negative)

Answer 13

Cocco-bacilli causing a respiratory tract infection.

Question 14

List clinically significant bacteria which are not readily identified by gram staining and outline how they are identified

Answer 14

1. Mycobacterium spp; visualised by special stains e.g. ZN or auramine. Culture is needed for full identification of spp. Includes tuberculosis and leprosy.
2. Spirochaetes; long spiral shaped bacterianot easily visualised with light microscopy and difficult to culture. Serology, dark ground microscopy or immunofluoresence. Includes lyme disease.
3. Chlamydia; Cultured in cell lines no agar and diagnosed via serology. Causes respiratory, ophthalmic and genital tract infection.