Bacterial Ocular Infections - Practice Questions

Characteristics of S. Aureus

-most virulent Staphylococcus
-only Staph strain to ferment mannose and grow in a high salt environment (7.5% NaCl)
-part of normal flora in the upper respiratory tract

S. Aureus Virulence Factors - Surface Components

-Capsule: antiphagocytic
-Protein A: inhibits complement fixation, opsonization, and ADCC by binding Fc portion of IgG
-Lipoteichoic acid and teichoic acids: promote adherence to mucosal surfaces and persistence in tissues binding to fibronectin

S. Aureus Virulence Factors - Enzymes

-Catalase: reduces phagocytic killing by converting H2O2 → H2O
-Penicillinase (β-lactamase): confers antibiotic resistance by clearing β-lactam ring of penicillins

S. Aureus Virulence Factors - Toxins

Toxic Shock Syndrome: acts as a superantigen → promotes massive cytokine release (causing TSS)

Which species of staphylococcus are coagulase negative?

-Staphylococcus epidermidis
-Staphylococcus saprophyticus

Which species of staphylococcus is a frequent cause of urinary tract infection?

Staphylococcus saprophyticus

Identification Tests for Staphylococcus aureus

-1. Catalase: separates Staphylococcus (+) from Streptococcus and Enterococcus (−)
-2. Coagulase: separates S. aureus (+) from other species of Staphylococcus (−)
-3. Protein A: separates S. aureus (+) from other species of Staphylococcus (−)
-4. Hemolysis: S. aureus is β-hemolytic
-5. Mannitol Salt Agar: S. aureus can ferment mannose and survive in a high salt environment (7.5% NaCl)

Characteristics and Mode of Transmission of P. aeruginosa

-most common clinically significant pseudomonas
-ubiquitous in the environment and in most reservoirs → infection usually begins where moisture accumulates
-resistant to soap, many disinfectants, and many antibiotics → but NOT drying

Identification of P. aeruginosa

-gram (-), oxidase (+), catalase (+), aerobic rod with 1-3 flagella
-non-lactose fermenting
-simple growth requirement
-characteristic fruit odor
-blue-green pigment (pyocyanin)
-β-hemolytic on blood agar

P. aeruginosa Virulence Factors - Surface Components

-Capsule: antiphagocytic and promotes adherence to epithelium
-Pili: promotes adherence to epithelium
-Biofilm: production occurs when sufficient bacteria trigger quorum sensing
-Antibiotic & Disinfectant Resistance

P. aeruginosa Virulence Factors - Enzymes

-Elastase & Phospholipase: mediate tissue destruction and inhibit neutrophil function
-Endotoxin exoenzyme S: prevents phagocytic killing

P. aeruginosa Virulence Factors - Toxins

-Exotoxin A: disrupts host protein synthesis
-Pigments (pyocyanin, pyoverdin): produce toxic forms of oxygen, stimulate cytokine release, regulate toxin secretion

P. aeruginosa Gram Stain

gram (-) → stains red

P. aeruginosa on Blood Agar

-demonstrates β-hemolysis → complete clearance
-greenish pigment
-grape-like odor

Treatment of Pseudomonas Keratitis

-Fortified Aminoglycosides (Tobramycin)
-β-Lactam antibiotic (Ceftriaxone)
-Ciprofloxacin (Ciloxan) & Moxifloxacin (Moxeza)

Which gram-positive, catalase negative, diplococcus, beta-hemolytic bacterium causes pseudomembrane bacterial conjunctivitis?

Streptococcus pyogenes

Which gram-positive, catalase negative, diplococcus, alpha-hemolytic bacterium causes dacryoadenitis?

Streptococcus pneumoniae

Which gram-negative, bacillus/rod-shaped bacteria is the major cause of contact-lens induced bacterial keratitis?

Pseudomonas aeruginosa

Which gram-positive bacterium is catalase positive and coagulase positive?

Staphylococcus aureus

Which bacterium has no cell wall?

Mycoplasma pneumoniae

Which bacterium causes cat scratch disease?

Bartonellae henselae

Which gram-negative coccobacillus bacteria causes hyperacute conjunctivitis?

Haemophilus influenzae
Haemophilus aegyptius

Which strain/serotype of Chlamydia trachomatis causes adult inclusion conjunctivitis?

Serotype D-K via sexual transmission or contact with bodily fluid

Which strain/serotype of Chlamydia trachomatis causes trachoma?

Strains A-C via contact with infected body fluids