Microbiology of Respiratory System

Microbiology of Respiratory system

Dr. Hala Al Daghistani

RTS is consists of :

TheUpper Respiratory Tractincludes the nose and nasal passages, paranasal sinuses, the pharynx, and the portion of the larynx above the vocal folds (cords). The LowerRespiratory Tractincludes the portion of the larynx below the vocal folds, trachea, bronchi and bronchioles, alveoli.

Upper respiratory system

·  Purpose: to take in warm and moisten air

·  Most common site of infection

Lower respiratory system

·  Purpose: ventilation, gas exchange

Barriers to RTS infections

·  Nasal hair

·  Muco-ciliary

·  Coughing

·  Normal biota

·  Secretory immunoglobulin

·  Defensins (are smallcysteine-richcationicproteins active againstbacteria,fungiand many enveloped and nonenvelopedviruses. Cells of the immune system (neutrophils granulocytesand almost all epithelial cells)contain these peptides to assist in killingphagocytosedbacteria. Most defensins function by binding to themicrobialcell membrane, and, once embedded, forming pore-like membrane defects that alloweffluxof essential ions and nutrients.

·  Phagocytic inflammatory cells

Common Nasopharyngeal and Oropharyngeal Organisms Isolated from the Normal Host

Bacteria

Usually present

·  Streptococcus mitis and other alpha-hemolytic Streptococci

·  Non–group A beta-hemolytic streptococci

·  Streptococcus pneumoniae

·  Streptococcus pyogenes

·  Streptococcus salivarius

·  Bacteroides spp.

·  Fusobacterium spp.

·  Porphyromonas spp

·  Coagulase-negative staphylococci

·  Neisseria spp.

·  Nonhemolytic streptococci

·  Diphtheroids

·  Micrococcus spp.

·  Eikenella spp.

·  Capnocytophaga spp.

Occasionally present

·  Haemophilus influenzae

·  Haemophilus parainfluenzae

·  Peptostreptococcus

·  Actinomycetes.

·  Staphylococcus aureus

·  Mycoplasma

Fungus

·  Candida spp.

Virus

·  Herpes simplex

Selected Pathogens in the Respiratory Tract

Primary Pathogens

Ø  Streptococcus pneumoniae

Ø  Group A β-hemolytic streptococci

Ø  Neisseria meningitidis

Ø  Neisseria gonorrhoeae

Ø  Bordetella pertussis

Ø  Mycobacterium tuberculosis

Ø  Legionella pneumophila

Ø  Toxin-producing Corynebacterium diphtheriae

Ø  Mycoplasma pneumoniae

Ø  Chlamydia trachomatis

Possible Pathogens

Ø  Acinetobacter spp.

Ø  Enterics and other gram-negative bacilli

Ø  Fungi

Ø  Nocardia spp.

Ø  Staphylococcus aureus

Ø  Haemophilus influenzae

Ø  β-Hemolytic streptococci, non–group A

Ø  Moraxella

Ø  Mycobacterium spp.

Ø  Actinomycetes

Group A Streptococci (Streptococcus pyogenes)

Morphology and growth

Ø  Group A Streptococci typically appear in purulent lesions or broth cultures as spherical or ovoid cells in chains of short to medium length. On blood agar plates, colonies are usually compact, small, and surrounded by a zone of β- hemolysis

Ø  Hemolysis is caused by either of two hemolysins, Streptolysin S and the oxygen-labile Streptolysin O, both of which are produced by most group A strains.

Ø  Strains that lack Streptolysin S are β- hemolytic only under anaerobic conditions, because the remaining streptolysin O is not active in the presence of oxygen.

Structure

M Protein

Ø  The M protein itself is a fibrillar coiled-coil molecule. The specificity of the more than 80 serotypes of M protein is determined by variations in the amino sequence of the aminoterminal portion of the molecule.

Ø  Because of its location, this part of the M protein is also the most available to immune surveillance.

Ø  Biologic functions of M protein includes both antigenicity and the capacity to bind other molecules such as fibrinogen, serum factor H, and Ig.

Other Surface Molecules

1.  A fibronectin binding Protein F

2.  IgG binding protein has the capacity to bind the Fc portion of antibodies (same as staphylococcal protein A). This could interfere with opsonization by creating a covering of antibody molecules on the Streptococcal surface that are facing the “wrong way.”

3.  Group A streptococci may have a hyaluronic acid capsule

Streptococcus pyogense Diseases

Pharyngitis

Ø  Group A streptococci are the most common bacterial cause of pharyngitis in school-age children 5 to 15 years of age.

Ø  Transmission is person to person from the large droplets produced by infected persons during coughing, sneezing, or even conversation.

Ø  Asymptomatic carriers (1%) may also be the source particularly if colonized in the nose as well as the throat.

PATHOGENESIS

Ø  M protein, protein F, and other fibronectin-binding proteins both are required for invasion of epithelial cells.

Ø  M protein plays an essential role in resistance to phagocytosis.

Ø  As a second antiphagocytic mechanism the C5a peptidase that inactivates C5a and thus blocks chemotaxis of polymorphonuclear neutrophils (PMNs) and other phagocytes to the site of infection.

Ø  Although the hyaluronic acid capsule contributes to resistance to phagocytosis, the mechanisms involved are unknown.

Ø  M protein supports nasopharyngeal cell adherence

Streptococcal Pharyngitis

• The illness is characterized by acute sore throat, malaise, fever, and headache.
• Infection typically involves the tonsillar pillars, and soft palate, which become red, swollen, and covered with a yellow-white exudate.
• The cervical lymph nodes that drain this area may also become swollen and tender.
• Group A streptococcal pharyngitis is usually self-limiting. Typically, the fever is gone by the third to fifth day, and other manifestations subside within 1 week.
• Occasionally the infection may spread locally to produce peritonsillar or retropharyngeal abscesses, otitis media, suppurative cervical adenitis, and acute sinusitis.
• Rarely, meningitis, pneumonia, or bacteremia with metastatic infection in distant organs.

DIAGNOSIS

• A direct Gram-stained smear of the throat
• Lancefield grouping using agglutination methods.
• An indirect method based on the susceptibility of Streptococcus beta hemolytic -group A strains to Bacitracin
• Several serologic tests have been developed to aid in the diagnosis of a previous infection with group A streptococcal group. They include the ASO, anti-DNAase B, and some tests that combine multiple antigens.

PREVENTION

• Penicillin prophylaxis is used to prevent recurrences of ARF during the most susceptible ages (5 to 15 years).
• Patients with a history of Acute Rheumatic Fever receive antimicrobial prophylaxis while undergoing procedures known to cause transient bacteremia, such as dental extraction.
• Vaccines using epitopes of the M protein molecule, which would provide protection against acute infection are in development.

Haemophilus influenzae

The Haemophilus Species

-  This is a group of small, gram-negative, pleomorphic bacteria that require enriched media, usually containing blood or its derivatives.

-  Haemophilus influenzae type b is an important human pathogen

Haemophilus influenzae

-  Haemophilus influenzae is found on the mucous membranes of the URT in humans (NF).

-  It is an important cause of meningitis in children and causes upper and lower respiratory tract infections in children and adults.

Morphology and Identification

A. Typical Organisms

- Short coccoid bacilli, sometimes occurring in pairs or short chains (pleomorphic)

- have a definite capsule.

B. Culture

- Chocolate agar, sheep blood agar around colonies of staphylococci (“Satellite phenomenon”).

- Haemophilus haemolyticus and Haemophilus parahaemolyticus are hemolytic variants of H. influenzae and Haemophilus parainfluenzae, respectively.

C. Growth Characteristics

- Factor X acts as hemin; factor V is Nicotinamide Adenine Nucleotide (NAD) or other coenzymes.

- Colonies of Staphylococci on sheep blood agar cause the release of NAD, yielding the satellite growth phenomenon.

- serotyping on the basis of capsular polysaccharides

Antigenic Structure

-  Encapsulated H. influenzae contains capsular polysaccharides of one of six types (a–f).

-  The capsular antigen of type b is a polyribitol ribose phosphate (PRP).

-  Most H. influenzae organisms in the normal microbiota of the URT are not encapsulated.

-  The somatic antigens is Lipooligosaccharides (LOS)

H. influenza Disaeses

-  H. influenzae type b enters by way of the respiratory tract. Encapsulated organisms may reach the bloodstream and be carried to the meninges and joints.

-  H. influenzae also produces common but less fulminant infections of the bronchi, respiratory sinuses, and middle ear. The latter are usually associated with nonencapsulated strains.

-  H. influenzae type b was the most common cause of bacterial meningitis in children age 5 months to 5 years in the United States.

Invasive Disease

-  For unknown reasons, H. influenzae strains commonly found in the normal flora of the nasopharynx occasionally invade into deeper tissues. Bacteremia then leads to spread to the CNS and metastatic infections at distant sites such as bones and joints.

-  Systemic spread is typical only for capsulated H. influenza strains, and over 90% of invasive strains are type b.

-  Attachment to respiratory epithelial cells mediated by pili, adhesins. No known exotoxins.

-  Once past the mucosal barrier, the antiphagocytic capsule confers resistance to opsonophagocytosis

-  Endotoxin in the cell wall is toxic to ciliated respiratory cells

Localized Disease

·  Nonencapsulated H. influenzae produce disease under circumstances including the middle ear, sinuses, or chronic bronchi.

Acute Epiglottitis

·  Acute epiglottitis is a dramatic infection in which the inflamed epiglottis and surrounding tissues obstruct the airway.

·  Hib is one of a number of causes. The onset is sudden, with fever, sore throat, hoarseness, cough, and rapid progression to severe prostration within 24 hours.

·  The hallmark of the disease is an inflamed, swollen, cherry-red epiglottis that protrudes into the airway and can be visualized on lateral x-rays.

EPIDEMIOLOGY

·  H. influenzae can be found in the normal nasopharyngeal flora of 20 to 80% of healthy persons, depending on age, season, and other factors.

·  Most of these are nonencapsulated, but capsulated strains, including Hib, are not rare.

·  Cases of epiglottitis and pneumonia tend to peak in the 2 to 5 y age. Over 90% of these cases are due to the single serotype, Hib.

IMMUNITY

·  Immunity to Hib infections has long been associated with the presence of anticapsular (PRP) antibodies, which are bactericidal in the presence of complement.

·  The infant is usually protected by passively acquired maternal antibody for the first few months of life. Thereafter the presence of actively acquired antibody increases with age; it is present in the serum of most children by 10 years of age.

·  Thus, systemic H. influenzae infections (meningitis, epiglottitis, cellulitis) are rare in adults.

·  Like many polysaccharides, Hib PRP behaves as a T cell–independent antigen. B cells mount the primary response without significant involvement of helper T cells.

·  Conjugation of PRP to protein dramatically improves the immunogenicity by eliciting the T-cell responses typical of protein antigens while preserving the specificity for PRP, even in infants.

Diagnostic Laboratory Tests

A. Specimens

sputum, pus, blood, and spinal fluid for smears and cultures

B. Direct Identification: immunologic detection of H. influenzae antigens in spinal fluid.

C. Culture

-  Chocolate agar

-  Blood agar with V factor

Epidemiology, Prevention, and Control

-  Encapsulated H. influenzae type b is transmitted from person to person by the respiratory route.

-  Two conjugate vaccines are available for use:

1. PRP-OMPC (capsule polysaccharide linked to outer membrane protein complex), the outer membrane protein complex of Neisseria meningitidis serogroup B.

2.  PRP-T (polyribitol ribose phosphate and tetanus toxiod)