TABLE 2.

Summary of key epidemiologic and virulence factors for ocular strains of P. aeruginosa, S. aureus, and S pneumoniae.

Pathogen	P. aeruginosa	S. aureus	S. pneumoniae
Key epidemiologic features	Ubiquitous; most common cause in contact-lens microbial keratitis Cytotoxic (ExolU+), invasive (ExoS−), or neither; epidemiologic composition in most locations unknown	Ubiquitous; common worldwide Majority (>90%) are methicillin-sensitive, but growing proportion of MRSA isolates particularly in tertiary care centers	Ubiquitous; most common in low-to-middle income countries Most corneal isolates characterized as encapsulated invasive strains
Minimum infectious dose in cornea (CFUs)	~50	~100	Unknown
Cell surface features	Lipopolysaccharide (LPS) Pili (fimbriae) Flagella Alginate Glycocalyx (biofilm)-forming peptides Type III secretion systems	Lipoprotein Lipoteichoic acid (LTA) MSCRAMMs including collagen, elastin, fibronectin, fibrinogen, and laminin-binding proteins, as well as clumping factor Peptidoglycans Protein A	Lipoteichoic acid (LTA) Peptidoglycans Polysaccharide capsule Pneumococcal adherence and virulence factors A and B (PavA and PavB) Pneumococcal choline-binding protein Pneumococcal surface antigen A (PsaA) Pneumococcal surface protein A (PspA) Pneumococcal surface protein C (PspC) Protein A
Secreted virulence factors	Exotoxin A Hemolysins Large exoprotease Leucocidin Metalloproteinases (Las A protease, Las B protease, alkaline protease, modified protease) Phospholipases Protease IV P. aeruginosa small protease (PASP)	α-toxin Catalase Coagulase DNAse Hemolysins (α-, β-, δ-, and γ- toxins) Hyaluronidase Leucocidin Staphylokinase Superantigens (enterotoxins A – D)	Autolysin Pneumolysin Reactive oxygen species (e.g., hydrogen peroxide) Secretory IgA protease
Antimicrobial resistance in ocular strains (in the US)	Low; <10% of ocular isolates resistant to fluoroquinolones and aminoglycosides (tobramycin). However, multi-drug resistant strains requiring last-line carbapenems and colistin are becoming of increasing concern.	High and rapidly accumulating: >30% of all strains resistant to fourth-generation fluoroquinolones) > 80% of MRSA resistant to fourth-generation fluoroquinolones ~ 10% MSSA resistant to fourth-generation fluoroquinolones No observed resistance to vancomycin	Low; <10% of ocular isolates resistant to fluoroquinolones.