Medical Microbiology II Lecture 2
-
Upload
carinajonglee -
Category
Documents
-
view
219 -
download
0
description
Transcript of Medical Microbiology II Lecture 2
-
MEDICAL MICROBIOLOGY II
LECTURE 2
Aerobic gram-positive rods
-
Mycobacteria
Belong to genus Mycobacterium Comprise of obligate pathogens,
opportunistic pathogens and saprophytes Characteristic:
Slender Straight or slightly curved Non-motile rods Aerobic Non-capsulated Non-sporing Do not stain easily but one stained they resist
decolourisation with dilute mineral salts (acid fast bacilli - AFB)
-
Mycobacteria of Clinical Importance
Pathogens
M. tuberculosis
M. africanum
M. bovis
M. leprae
M. ulcerans (M. buruli)
Diseases
Tuberculosis
Tuberculosis
Tuberculosis
Leprosy
Skin ulcers (Buruli ulcer)
-
Culture Media for Mycobacteria
1. Lowenstein-Jensen (LJ) glycerol medium Recommended medium for human type
tubercle bacilli The malachite green in the medium is
inhibitory to organisms other than mycobacteria
Also provides a colour contrast which makes it easy to recognise colonies even when they are small
Glycerol in the medium enhances the growth of M. tuberculosis but not that of M. bovis which may fail to grow on it
-
Culture Media for Mycobacteria
2. Lowenstein-Jensen (LJ) pyruvate medium
Medium used for the isolation of M. bovis
3. Dorset egg medium
Simple medium
4. Dubos medium
One of the earliest liquid medium
5. Selective Kirchner medium
6. Middlebrook 7H9 broth
-
Staining Characteristics
Known for their acid fastness
When they are stained with carbol fuchsin in the Ziehl-Neelsens (ZN) method, the bacilli are able to retain the red colour of the stain after being treated with mineral acid solution
This happens because the organisms contain a thick cell wall which is composed of waxes and lipids with high content of mycolic fatty acids
The heated stain is able to penetrate the thick cell wall
-
Staining Characteristics
They resist decolourisation with mineral acids so they are called acid fast bacilli
They stain poorly with Grams stain, appearing as Gram positive (shadow cells)
Degree of acid-fastness varies from species to species
-
Mycobacterium tuberculosis
Major cause of tuberculosis (TB) in human. Other causative agents of TB include M. bovis and M. africanum
M. tuberculosis was originally called Kochs bacillus after Robert Koch who first identified it in 1882
TB is contagious
Main route of the bacilli to enter the body is by inhalation of droplets or dust particles containing the bacilli
-
Tuberculosis
There are different forms of TB:1. Pulmonary TB
In primary infection, the bacilli lodge in the pulmonary alveoli and the surrounding lymph glands resulting in lesions
These lesions have an accumulation of fluid and white blood cells around it
The lesion may heal or become productive. In the productive stage, infected lung tissue is
being liquefied and broken down into a cheese-like mass (caseation)
-
Tuberculosis
These yellow pieces of caseation material contain large number of bacilli and are coughed out in sputum by the patient
The infection creates cavities in the lungs making breathing difficult
Symptoms in adults: chronic cough with production of mucopurulent sputum which may contain blood, loss of weight, fever, tiredness, chest pain, anaemia and night sweats (during sleep)
-
Pulmonary Tuberculosis
-
Tuberculosis
Symptoms in children: weight loss and enlargement of lymph glands which may cause obstruction of the bronchi and emphysema
Complications of pulmonary TB: pleural effusion, lung collapse, acute miliary TB and tuberculous meningitis, especially in children
-
Tuberculosis
2. Non-pulmonary TB
I. Renal and urogenital TB The bacilli get to the kidney and genital tract
via the blood stream
Suspicion of TB will start when repeated urine specimens show pus cells with negative urine culture
Symptoms: frequent urination, haematuria, fever
Complications: pelvic inflammatory disease and infertility
-
Tuberculosis
II. Miliary tuberculosis Rupture of a site of primary infection into a
vein results in a spread of infection to other parts of the body
This is referred to as miliary TB
Symptoms: fever
Chest x-ray will show wide spread of fine nodules and usually low white cell count
Complications: liver, spleen and lymph glands are enlarged and meningitis may occur
-
Miliary Tuberculosis
-
Tuberculosis
III. Tuberculous meningitis
Occurs most commonly in infants and young children following a primary TB infection
Fatality is high unless treated promptly
When CSF contains high lymphocytes count, TB should be excluded
-
Laboratory Diagnosis
Specimens: sputum, pleural effusion, CSF and urine
Microscopy: Gram stain: not routinely done, not good
staining Ziehl-Neelsen stain:
Smears made directly from specimens and stained by ZN method
Bacilli are stained red against a blue-green background
At least 100 fields should be examined before a negative smear report is given
-
Laboratory Diagnosis
Should spend 10 - 15 mins on one slide
A positive report is given only if two or three typical bacilli are seen
Bacilli are counted and report as the number of bacilli per high power field
-
Ziehl-Neelsen Stained M. tuberculosis
-
Laboratory Diagnosis
Culture
A definitive diagnosis of tuberculosis is dependent on the isolation and identification of M. tuberculosis by culture
All laboratories send their specimens for culture and sensitivity to a tuberculosis reference centre where full and controlled facilities are available for isolation and identification of Mycobacterium species
For Sarawak: Klinik ATAS, Jln Masjid
-
Colonies of M. tuberculosis
-
Aerobic Gram Positive Rods
Aerobic Gram positive rods include members of the genera:
1. Corynebacterium
2. Listeria
3. Erysipelothrix
4. Lactobacillus
5. Bacillus
-
Corynebacterium
Consists of Gram positive straight or curved rods
May be club shaped at one pole
Non-motile, non-acid fast, non-sporing and may stain irregularly (appear beaded or barred)
Normally found in soil, plants and animals
Non-pathogenic strains form part of normal flora of skin, upper respiratory tract, urinary tract and conjunctiva
-
Corynebacterium diptheriae
Has 3 subspecies: C. diptheriae gravis, C. diptheriae intermedius and C. diptheriae mitis
Causes nasal, nasopharyngeal and tonsilar diphtheria
Infection is by inhalation of infected droplets or by contact with contaminated objects
Carriers are a major source of transmission as they secrete large numbers of the bacilli in the nose
-
Diptheria
An acute infectious disease caused by C. diptheriae
Spreads through respiratory droplets (such as those produced by a cough or sneeze) of an infected person or carrier (someone who carries the bacteria but has no symptoms)
Can also be spread by contaminated objects or foods (such as contaminated milk)
-
Diptheria
The bacteria most commonly infects the nose and throat.
The throat infection causes a gray to black, tough, fiber-like covering - characteristic of diptheria
The covering (pseudomembrane) can mechanically obstruct the passage of air in the larynx and cause death by asphyxiation
-
Diptheria
In some cases, diphtheria may first infect the skin, producing skin lesions
Once infected, dangerous substances called toxins, produced by the bacteria, can spread through your bloodstream to other organs, such as the heart, and cause significant damage.
-
Symptoms
Usually occurs 2 to 5 days after infection
Bluish colouration of the skin
Bloody, watery drainage from nose
Breathing problems (difficulty breathing, rapid breathing
Chills, fever
Croup-like (barking) cough
Drooling (suggests airway blockage is about to occur)
-
Symptoms
Painful swallowing
Skin lesions (usually seen in tropical areas)
Hoarseness , sore throat (may range from mild to severe)
Note: There may also be asymptomatic
-
Diptheria
Other major complications:
1. Acute circulatory failure - peripheral or cardiac
2. Septic conditions - pneumonia, otitis media
3. Post-diptheric paralysis - may resolve spontaneously
-
Laboratory Diagnosis
Specimens: throat, nasal or nasopharyngeal swab or skin swab (cutaneous diptheria)
Diagnosis usually made clinically, laboratory just needs to confirm the clinical diagnosis
Microscopy
Gram stain: not useful, generally pleomorphic with club shaped ends and are arranged in palisades or the typical Chinese lettering forms
Alberts stain: the organism stain green while the granules are darkly stained
-
C. diptheriae Gram stained
-
Laboratory Diagnosis
Culture
Media used in the isolation of C. diptheriae are blood agar, Hoyles tellurite medium and Loefflers serum slope
Blood agar is used to exclude haemolytic Streptococcus which may cause similar lesion
Hoyles tellurite medium is selective for C. diptheriae. It reduces tellurite and produces grey to black colonies 0.5 - 2 mm in diameter after 40 hrs
-
Laboratory Diagnosis
Loefflers serum agar enhances the production of volutin granules
This medium promotes the growth of C. diptheriae within 4 - 6 hrs
Identification tests
Biochemically: ferments glucose and maltose; catalase and nitrate positive; urease negative
Antibiotic sensitivity
Usually sensitive to penicillin, erythromycin and lincomycin
-
Bacillus
Consist of large aerobic spore-bearing rods
Gram positive bacilli which may sometimes be arranged in long chains
Most species are motile
Main pathogen is B. anthracis (anthrax bacillus)
Other species that may cause infection include B. cereus and B. subtilis
Some other species are saprophytes
-
Bacillus anthracis
Bacillus anthracis is primarily a pathogen of herbivores and are excreted in the faeces, urine and saliva of infected animals
They form spores which can remain viable in the environment for years
B. cereus and B. subtilis are saprophytes found in soil, water, vegetation and foodstuffs such as meat, milk, cereals and spices
They form spores which can be found everywhere
-
Anthrax Caused by B. anthracis
1. Cutaneous anthrax
starts as a single painless blister (malignant pustule)
Infection caused by spores entering through damaged skin
The spores germinate in the tissue at the site of entry and spread via the lymphatics to blood stream causing septicaemia, toxaemia and death
-
Anthrax Herdsmen, butchers and those involved in
treating of animal skins are most at risk
-
Anthrax
2. Pulmonary anthrax
Caused by inhalation of the organism or spores
Causes a severe haemorrhagic condition in the bronchi and lungs which may result in septicaemia and death
Condition is referred to as wool sorters disease as those engaged in wool and hides treatment are usually infected
-
Anthrax
3. Enteric anthrax
Caused by ingestion of contaminated foodstuffs such as meat and milk
Severe form of gastroenteritis which progresses to septicaemia, toxaemia and death
Meningitis is a complication of anthrax septicaemia
-
B. cereus
Causes food poisoning which may result from eating of either contaminated cooked rice or cereals, or meat
The symptoms are onset of vomiting and diarrhoea within a few hours or up to 24 hours depending on the kind of contaminated food eaten
Pneumonia, broncho pneumonia and wound infections - common
-
B. subtilis
Reported to cause meningitis, endocarditis and other infections in debilitated and immunosuppressed patients
-
Laboratory Diagnosis Specimens: pus or fluid from the pustule,
sputum, blood, CSF and swabs from cutaneous lesions (all should be treated as HIGHLY INFECTIOUS)
Microscopy
Seen as Gram positive rods, 5 - 8 x 1.5 m with square or round ends, no spores
Polychrome methylene blue staining show capsulated large bacilli mostly in chain, capsule material stains purple - spores are centrally placed
-
B. anthracis Gram stained
-
Laboratory Diagnosis
Motility
B. anthracis is non-motile while the rest are generally motile
Culture
Bacillus grow well on most culture media, aerobically at optimum temperature of 35 -37 C (range of 12 - 45 C)
Blood agar: B. anthracis produces large (2 - 3 mm in diameter), grey, opaque irregular colonies with wavy edges (Medusa head)
-
B. anthracis on blood agar
-
Laboratory Diagnosis
Colonies are non-haemolytic or slightly haemolytic
When grown in the presence of a high concentration of carbon dioxide, it produces smooth mucoid colonies
Broth cultures of B. anthracis contain thick pellicles on the surface of the medium, with no turbidity.
Other species produce turbidity in broth culture
-
Laboratory Diagnosis
-
Laboratory Diagnosis
Identification tests
Biochemically: ferments glucose, maltose, sucrose and trehalose with acid and no gas production
Catalase positive, nitrate positive, liquefies gelatine slowly in an inverted fir tree pattern
Antibiotic sensitivity
Sensitive to penicillin, tetracycline, chloramphenicol and cotrimoxazole
-
THE END