Flashcards in Microbiology L05 Gram negative cocci (based on lecture outcomes) Deck (24)
What are TWO properties of Mycobacterium that are relevant to the clinician?
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
What is the ZN stain and what does AFB signify?
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
How are mycobacteria classified?
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.)
What is the impact of TB on the world’s population?
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
How is M. tuberculosis transmitted?
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
What happens in human host after infection with M. tuberculosis?
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
What are some potential disease manifestations following M. tuberculosis infection?
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
What is the relationship between TB and HIV?
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
What is the Mantoux test?
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
What drugs are used to treat TB? What is MDR-TB? XDR-TB?
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
What tissues are infected by M. leprae and what is the resulting clinical manifestation?
Skin is main tissue affected and results in leprosy
What does atypical mycobacteria, NTM, MOTB, MOTT mean?
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
What is the Gram stain appearance of Neisseria meningitides?
Gram negative diplococci – flattened to resemble kidneys
What is the purpose of the polysaccharide capsule on N. meningitides?
Meningococci divided into serogroups based on different antigenic properties of the polysaccharide capsules
What range of illnesses are caused by N. meningitidis and what are clinical features of meningitis and meningococcaemia?
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)
Why is N. meningitidis infection so serious?
Quickly fatal once infected AND illness may have NON-SPECIFIC features at early presentation
How is N. meningitides carried and what features of prophylaxis does it have?
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.
What is the gram stain appearance of Mycobacterium?
Gram negative (pink) rods. However, the ZN stain makes them red
What are the clinical features of TB infection?
Very gradual, non specific onset:
How do you diagnose TB?
Mantoux - previous exposure, not if it's active or latent though
Interferon gamma release assay
Chest X-ray to find pulmonary lesion
What are some Zoonotic causes of fever?
Toxoplasma, associated with cats
Rickettsia (fleas, mites & lice)
How many respiratory samples do you need when looking for TB?
3 early morning ones. Need at least 3 to be sure of active infection
What antibiotics do you use for resistant TB?
Quinolones, macrolides & streptomycin