Microbio Lec 6 - Strep Pneumoniae

MICROBIOLOGY LECTURE 6 – Streptococcus pneumoniae Notes from Lecture USTMED ’07 Sec C - AsM

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STREPTOCOCCUS PNEUMONIAE

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Gram positive lancet shaped cocci arranged in pairs (diplococcis) or in short chains

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Possesses a capsule of polysaccharide that permits typing with specific antisera Produces alpha hemolysis in blood agar Lysed by surface active agents such as bile or deoxycholate Growth is inhibited by optochin Normal inhabitants of the upper respiratory tract of humans Most common cause of pneumonia and otitis media An important cause of meningitis and bacteremia/sepsis

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CLINICAL FINDINGS

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Pneumococcal pneumonia a. infections are caused by aspiration of the endogenous oral organisms b. most commonly associated with an antecedent viral respiratory disease such as influenza and measles c. abrupt onset of fever, chills, chest pain and productive cough with blood tinged sputum or rusty sputum followed by a crisis on days 7 to 10 d. pleural effusion in seen in 25% of patients; empyema is a rare complication

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Sinusitis and otitis media o preceded by a viral infection of the upper respiratory tract, leading to infiltration with polymorphonuclear leukocytes and obstruction of the sinuses and middle ear Meningitis o Spread of S. pneumoniae into the central nervous system can follow bacteremia, infections of the ear and sinuses, or head trauma with communication between the subarachnoid space and nasopharynx.

ANTIGENIC STRUCTURE

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Polysaccharide capsule or specific soluble substance a. virulence factors – interferes with phagocytosis and favor invasiveness b. specific antibody to capsule i. Opsonizes the organism ii. Facilitates phagocytosis iii. Promotes resistance

c.

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Quellung reaction- swelling of capsule when type specific antiserum is added to the organism d. elicits a B cell response C-substance a. a carbohydrate in the cell wall

b.

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reacts with a normal serum protein made by the liver – C-reactive protein(CRP)

CRP – is a nonspecific indicator of inflammation and elevated in response to the presence of many organisms

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PATHOGENESIS

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The most important virulence factor is the capsular polysaccharide, and anticapsular antibody is protective.

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Lipoteichoic acid, which activates complement and induces inflammatory cytokine production, contributes to the inflammatory response and to the septic shock syndrome that occurs in some immunocompromised patients. Pneumolysin, the hemolysin that causes alpha hemolysis, may also contribute to the pathogenesis. Pneumococci produce IgA protease that enhances the organism’s ability to colonize the mucosa of the upper respiratory tract. Pneumococci multiply in tissues and cause inflammation. When they reach the alveoli, there is outpouring of fluid and red and white blood cells, results in consolidation of the lung. During recovery, pneumococci are phagocytized, mononuclear cells ingest debris, and consolidation resolves.

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Streptococcus pneumoniae virulence factors

Alcohol or drug intoxication or other cerebral impairment that can depress the cough reflex and increase aspiration of secretions Abnormality of the respiratory tract (eg. Viral infections), pooling of mucus, bronchial obstruction and respiratory tract injury and movement of the mucociliary blanket) Abdominal circulatory dynamics (eg. Pulmonary congestion and heart failure) Splenectomy Certain chronic diseases such as sickle cell anemia and nephrosis

Bacteremia o occurs inb 25% to 30% of patients with pneumococcal pneumonia and more than 80% of patients with meningitis

Laboratory Diagnosis 1. Microscopy

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Gram stain of respiratory secretions. Gram positive lancet shaped diplococci suggestive of Streptococcus pneumoniae

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Culture

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Colonies of Streptococcus pneumoniae on 5% sheep blood agar. Young colonies are round with complete edges, somewhat mucoid, and about 1 mm in diameter urrounded by a zone of alpha hemolysis

Colonies of Streptococcus pneumoniae in 5% sheep blood agar Slightly older colonies show the central indentation caused by the Easily induced autolysis.

Occasionally, colonies may simply flatten out a as they age. Factors that lower resistance and predispose to pneumococcal infection

Colonies of Streptococcus pneumoniae on chocolate agar TREATMENT

Medium was incubated in CO2, resultinn in fairly large zones of alpha hemolysis

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Colonies are quite flat on Chocolate agar

Most pneumococci are susceptible to penicillins and erythromycin. Severe pneumococcal infections – penicillin G Mild pneumococcal infections –oral penicillin V Penicillin allergic patients- erythromycin or its derivatives, azithromycin Penicillin resistant strains – vancomycin

PREVENTION

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Polyvalent (23 type) polysaccharide vaccine a. provides long lasting protection(at least 5 years) b. should be given to  Elderly persons(over 65 years)

Optochin Susceptibility Test

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Zone of greater than or equal to 14 mm in diameter is resumptive identification of Streptococcus pneumoniae. If less that 14 mm in diameter, confirm with bile solubility test. A paper disk containing optochin (ethylhydrocupreine Hydrochloride) is applied to the surface of an inoculated 5% sheep blood agar.

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Bile solubility test A drop of deoxycholate directly to The colonies.

2% is

sodium applied

Left – colonies remain intact Right – lysis of the colonies

After the bile has evaporated, Only the zone of hemolysis remains

Pneumococcal vaccine containing pneumococcal polysaccharide coupled to a carrier protein(diphtheria toxoid) as the immunogen a. contains the 7 most common pneumococcal serptypes b.

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Immunocompromised (splectomized)  Debilitated persons booster should be given to: i. Over 65 years who received the vaccine more than 5 years ago ii. People between the ages 2 and 64 who are asplenic, HIV infected, receiving cancer chemotherapy or receiving immunosuppressive drugs to prevent transplant rejection

effective in young children in preventing both bacteremic infections, such as meningitis, and mucosal infections like otitis media

Oral penicillin is given to young children with hypogammaglobulinemia or splenectomy because they are prone to pneumococcal infection and respond poorly to vaccine.

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Capsule swelling tests