Microbiology TB Flashcards Preview

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Flashcards in Microbiology TB Deck (37):
1

Classification

• M. tuberculosis (MTB) complex (Typical)
• MOTT (mycobacteria other than TB) (Atypical or Non-tuberculous mycobacterium (NTM)) – more difficult to manage.

2

MTB Complex

• M. tuberculosis
• M. bovis (inc. BCG)
• M. africanum
• M. microti
• M. canetti
• M. caprae
• M. pimmipedii

3

Non-cultivatable mycobacterium

• M leprae

4

Runyon Classification (1959) MOTT

Atypical mycobacterium
• I Photochromogens → Yellow pigment formed after exposure to light when colonies grown in the dark and take more than 7 days
• II scotochromogens → Yellow or orange pigment formed when colonies grown in the dark and take more than 7 days
• III Nonphotochromogens → Colonies are non-pigmented regardless of whether grown in the dark or light and take more than 7 days
• IV Rapid growers → colonis (pigmented or non- pigmented) that take less than 7 days

5

Rapid growers

Non Chromogens →
Chromogens
See page 110

6

1

Number of tubercle bacilli required to establish infection

7

10

Average number of people that get infected by a single case of pulmonary TB

8

15

Number of years for which the incidence of TB has been progressively increasing in the UK

9

20

Time in hours for M. tuberculosis, a slow growing mycobacterium, to replicate

10

130

Hours of exposure to a case of infections pulmonary TB needed to be sure of contracting TB infection

11

6,669

Number of cases of TB in the UK in 2001

12

2,500,000

Annual number of deaths due to TB globally

13

Mycobacterium Tuberculosis → Description

Human pathogen

14

Mycobacterium Tuberculosis →Transmitted by

Respiratory droplet (infectious dose: 1-10 bacilli)

15

Mycobacterium Tuberculosis →Adapted to

Intracellular survival within the human macrophage
• Latency/dormant/non-replicating persistence
• Allows lifelong infection

16

Mycobacterium Tuberculosis →Factors that promote progression to active disease

HIV
• At all CD4 counts
• More extrapulmonary disease
Immunosuppressive drugs (iatrogenic)
• High dose steroids
• Infliximab (anti-TNF w/ latent TB due to T-cells)
Age: very young; very old
Poor nutrition
Homelessness/alcohol/ IVDA/ poverty

17

Patients with active disease

Treat especially with infectious pulmonary tuberculosis

18

Vaccination

Limited and variable efficacy (UK vs India; prevents dissemination)
Age 12-14, or at birth if parents are from high-risk groups.

19

Diagnose people with latent tuberculosis infection and give preventative therapy

1 infections case infects 10 other people, of whom 1 will develop TB
Tuberculin skin test (Heaf): cross-reactivity of PPD with BCG
Contact tracing
New arrivals from high prevalence areas
Child contacts

20

Diagnosis →

Specimens
Procedures
Culture
Histology

21

Specimens

Sputum, gastric washings, bronchoalveolar lavage
Early morning urines
Biopsies

22

Procedures

Microscopy (result within 24 h; not all AFBs are TB)
• Ziehl-Neelson
• Auramine

23

Culture

Crucially important, but often negative)
Solid phase: Lowenstein-Jensen medium
Liquid phase: uptake and release of radiolabelled carbon
Drug sensitivities

24

Histology

Granulomata with central caseous necrosis

25

Drawbacks of Tuberculin Skin Test →

Poor specificity
Poor sensitivity
Operational drawbacks

26

Tuberculin Skin Test Poor specificity

Antigenic cross-reactivity of PPD with BCG and environmental mycobacteria

27

Tuberculin Skin Test Poor sensitivity

75-90% in active disease (lower in disseminated TB and HIV infection, unknown for latent infection.

28

Tuberculin Skin Test Operational drawbacks

Need for return visit
Operator variability (inoculation and reading)
Standardisation of reagent
Painful inflammation and scarring

29

In-vitro and in-vivo diagnostic tests

APC presenting mycobacterial antigens to memory T-cells
See diagram page 112

30

ELISPOT principle of test

RD1 contains the genes for ESAT6 (early secretory antigen target 6) and CFP10 (culture filtrate protein 16)
→ doesn’t give a false +ve w/ BCG as the genes aren’t in BCG bt +ve result = low risk popn
→ Can’t be –ve in patients w/ active disease

31

Definition resistant TB

MDR-TB (Multidrug Resistant TB) describes strains of tuberculosis that are resistant to at least the two main first-like TB drugs – isoniazid and rifampicin.

32

XDR-TB or Extensive Drug resistant TB (also referred to as Extreme Drug resistance)

Is MDR – TB that is also resistant to three or more of the six classes of second-line drugs.

33

MDR-TB

Resistance to anti-TB drugs in populations is a phenomenon that occurs primarily due to poorly managed TB care. Problems include incorrect drug prescribing practices by providers, poor quality drugs or erratic supply of drugs, and also patient non-adherence.

34

Risk assessment for MDR-TB

1. History of prior TB drug treatment, prior TB treatment failure
2. Contact with a known case of durg-resistant TB
3. Birth in a foreign country, particularly high-nicidence countries
4. HIV infection
5. Residence in London
6. Age profile, with highest rates between ages 25 and 44
7. Male gender

35

XDR-TB

1. First described in 2006
2. During 2000-20004, of 17, 690 TB isolates referred to international network of TB laboratories, 20% were MDR nd 2% were XDR
3. In addition, population-based data on drug susceptibility of TB isolates were obtained from the United States (for 1993-2004), Latvia (for 2000-2002), and South Korea (for 2004), where 4%, 19% and 15% of MDR TB cases, respectively, were VDR

36

Resistant TB →

A tuberculous cavity contains 107 to 109 bacilli. If mutations causing resistance to INH occurs in 1 in 106, and mutations causing RIF resistance occur in about 1 in 108, the probability of spontaneous MDR is 1 in 1014.

37

Treatment of resistant TB (non-MDR)→

)→ resistant to 1 or 2 → streptomycin.
Depends on site = initiation phase (4 drugs for 2 months), (2 drugs for 4 months).

4 drugs used:
1. Rifamapicin
2. Ioniaziad
3. Perizonamide
4. Ethanbutol

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