Microbiology L05 Gram negative cocci (based on lecture outcomes)

Question 1

What are TWO properties of Mycobacterium that are relevant to the clinician?

Answer 1

Slow growth rate – implications for timely detection and identification in diagnostic laboratory
Mycolic acid – fatty, hydrophobic component of cell wall → doesn’t take up Gram stain well, makes mycobacteria hardy and resistant to drying

Question 2

What is the ZN stain and what does AFB signify?

Answer 2

ZN stain is the Ziehl Neelsen stain – uses a strong carbol fuschin at a higher concentration to the Gram stain to visualize mycobacteria → carbol fuschin taken up by mycolic acid → bacteria contain mycolic acid resist decolourisation by acid-alcohol mix → add contrasting colour → visualisation
AFB – acid-fast bacilli – resist acid decolourisation step of ZN stain

Question 3

How are mycobacteria classified?

Answer 3

Practically classified into: Mycobacterium tuberculosis (TB), Mycobacterium leprae (leprosy) or non-tuberculous mycobacteria (NTM)
Mycobacteria grouped with genetically similar species into complexes (Mycobacterium tuberculosis complex, Mycobacterium avium complex etc.)

Question 4

What is the impact of TB on the world’s population?

Answer 4

Approximately 1/3 of entire population of the world is infected with this organism
In 2011, caused illness in 8.7 million people worldwide with 1.4 million deaths
Fell during the 20th century with living standard improvements but is now making a comeback in people with AIDS, urban homeless, immigrants

Question 5

How is M. tuberculosis transmitted?

Answer 5

Acquired by inhalation from another person with active pulmonary TB – transmitter sheds tubercule bacilli from lung in large amounts when coughing, sneezing, shouting, singing → persist in air of room for a while – ‘aerosol particles’ – 0.5-5.0 um. Small size gives particles ability to be inspired directly to alveolar level without being filtered out

Question 6

What happens in human host after infection with M. tuberculosis?

Answer 6

Majority of people (90%) of people infected with M. tuberculosis remain asymptomatic
10% will develop symptoms, 5% in 2 years (1° TB), 5% 2 years later (2° TB) due to reactivation of latent TB
10-25% of cases extrapulmonary

Question 7

What are some potential disease manifestations following M. tuberculosis infection?

Answer 7

Ghon focus – granulomatous inflammation, 1-1.5cm diameter, unilateral, midlung adjacent to pleura, central caseation necrosis → becomes fibrosed, walled off and later calcifies
Primary tuberculosis → after engulfment by macrophages and replication → go to lymph nodes and are disseminated around body (lymphatics, bone marrow, meninges, kidney)
Milliary TB – isolated organ affected (liver, spleen)
Secondary TB → later in life – apical fibrosis, calcification, pleural inflammatory effusion, empyema, airway spread to endotracheal TB

Question 8

What is the relationship between TB and HIV?

Answer 8

Patients with HIV are immunocompromised and have very low CD4+ T cells available to fight off infection. Thus it is MUCH easier for TB to survive and spread within the body → disease
If infected with both, patients had a 10% annual risk of developing active disease

Question 9

What is the Mantoux test?

Answer 9

Intradermal injection of a bleb purified preparation of M. tuberculosis antigen (PPD) → people infected with MTB will develop a local skin reaction measurable after 48hrs. Positive test does not differentiate between latent and active infection

Question 10

What drugs are used to treat TB? What is MDR-TB? XDR-TB?

Answer 10

First line drugs: isoniazid, rifampicin, pyrazinamide, ethambutol for 6 months
MDR TB – multidrug resistance – resistant to isoniazid and rifampicin
XDR TB – extensive drug resistance – resistant to all 1st line drug AND some others

Question 11

What tissues are infected by M. leprae and what is the resulting clinical manifestation?

Answer 11

Skin is main tissue affected and results in leprosy

Question 12

What does atypical mycobacteria, NTM, MOTB, MOTT mean?

Answer 12

Atypical mycobacteria – neither M. tuberculosis or M. leprae (other 3 are just versions of the same group)
NTM – non tuberculosis mycobacteria
MOTB – mycobacteria other than TB
MOTT – mycobacteria other than tuberculosis

Question 13

What is the Gram stain appearance of Neisseria meningitides?

Answer 13

Gram negative diplococci – flattened to resemble kidneys

Question 14

What is the purpose of the polysaccharide capsule on N. meningitides?

Answer 14

Virulence factor

Meningococci divided into serogroups based on different antigenic properties of the polysaccharide capsules

Question 15

What range of illnesses are caused by N. meningitidis and what are clinical features of meningitis and meningococcaemia?

Answer 15

N. meningitides → rapidly fatal infection, meningitis, bacteraemia or both together can progress to shock, disseminated intravascular coagulation and pupura fulminans (necrosis of skin, digits, limbs)
Meningitis → fever, meningism (headache, neck stiffness, photophobia), pus in CSF
Meningococcaemia → fever, rash (petechiae evolving into purpura and ecchymoses)

Question 16

Why is N. meningitidis infection so serious?

Answer 16

Quickly fatal once infected AND illness may have NON-SPECIFIC features at early presentation

Question 17

How is N. meningitides carried and what features of prophylaxis does it have?

Answer 17

Carried in the nasopharynx of 5-15% of the healthy population, transiently or chronically – non-pathogenic strains – transmissible and prerequisite for system infection development – pathogenic strains have virulence factors allowing invasion
Infection with one serogroup provides protection against re-infection with that same group. Vaccination also provides protection.

Question 18

What is the gram stain appearance of Mycobacterium?

Answer 18

Gram negative (pink) rods. However, the ZN stain makes them red

Question 19

What are the clinical features of TB infection?

Answer 19

Very gradual, non specific onset:
Fever
Night sweats
Tiredness
Weight loss
Persistant cough

Question 20

How do you diagnose TB?

Answer 20

Mantoux - previous exposure, not if it’s active or latent though
Quantiferon gold
Interferon gamma release assay
Chest X-ray to find pulmonary lesion

Question 21

What are some Zoonotic causes of fever?

Answer 21

Toxoplasma, associated with cats
Bartonella
Q fever
Rickettsia (fleas, mites & lice)
Brucella

Question 22

How many respiratory samples do you need when looking for TB?

Answer 22

3 early morning ones. Need at least 3 to be sure of active infection

Question 23

What antibiotics do you use for resistant TB?

Answer 23

Quinolones, macrolides & streptomycin

Question 24

Describe the progression on meningitis from onset

Answer 24

Onset is nonspecific – fever, muscle or joint aches, RUBELLIFORM rash, acrocyanosis (median onset 7-12 hours). Severe muscle aches – not usually associated with viral infections, so should be a warning sign.
Progresses to petechial rash, neck stiffness photophobia, vomiting and drowsiness (median onset 13-22 hours)
Meningitis – 5% mortality•
Septicaemia – 20% mortality