24.1 Types of Microorganisms & Pathways of Disease Transmission
Key Takeaways
- The four organism classes named in the blueprint are bacteria, viruses, fungi and protozoa.
- Gram-positive organisms retain crystal violet and appear purple; Gram-negative organisms appear pink after counterstaining.
- Adenovirus survives on dry surfaces for weeks and is resistant to alcohol, making contact transmission the dominant route in eye clinics.
- Only a handful of organisms, including Neisseria gonorrhoeae, can penetrate an intact corneal epithelium.
- Hand hygiene is the single most effective interruption of contact transmission in an ophthalmic clinic.
The four classes of microorganism
| Class | Structure | Ocular examples |
|---|---|---|
| Bacteria | Prokaryotic, no nucleus, cell wall, replicate independently | Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Pseudomonas aeruginosa, Haemophilus influenzae, Neisseria gonorrhoeae, Chlamydia trachomatis, Moraxella |
| Viruses | Not cells; nucleic acid in a protein coat; obligate intracellular parasites | Adenovirus, herpes simplex, varicella zoster, cytomegalovirus, enterovirus |
| Fungi | Eukaryotic; filamentous (moulds) or yeasts | Fusarium, Aspergillus (filamentous); Candida (yeast) |
| Protozoa | Single-celled eukaryotes | Acanthamoeba, Toxoplasma gondii |
Two therapeutic consequences follow from the definitions. Viruses replicate inside host cells, so antibacterial agents are useless against them and an antibiotic prescribed for adenoviral conjunctivitis achieves nothing except selecting for resistance. Fungi are eukaryotes like human cells, which is why antifungal treatment is slow and relatively toxic.
Gram staining
The Gram stain divides bacteria by cell wall structure and is the first result available from a corneal scraping.
| Result | Cell wall | Appearance | Ocular examples |
|---|---|---|---|
| Gram-positive | Thick peptidoglycan that retains crystal violet | Purple | Staphylococcus, Streptococcus, Bacillus, Corynebacterium |
| Gram-negative | Thin peptidoglycan with an outer membrane; loses crystal violet and takes the safranin counterstain | Pink | Pseudomonas, Haemophilus, Moraxella, Neisseria, Serratia |
Morphology is reported alongside: cocci (round), bacilli (rods), in clusters, chains or pairs. "Gram-positive cocci in clusters" means staphylococci; "Gram-negative diplococci" means Neisseria.
Other stains that appear in ophthalmic microbiology: Giemsa (intracytoplasmic inclusions of Chlamydia, and the cell type in conjunctival scrapings), calcofluor white and potassium hydroxide (fungal hyphae), acid-fast (Ziehl-Neelsen) for mycobacteria, and acridine orange.
Ocular defences
The eye is remarkably resistant to infection, and understanding why explains when infection happens.
- Lids and lashes provide a mechanical barrier and the blink sweeps debris away.
- Tears flush mechanically and contain lysozyme, lactoferrin, immunoglobulin A, defensins and complement.
- The corneal epithelium is a tight barrier; an intact epithelium is the single most important defence.
- Conjunctival lymphoid tissue mounts a local immune response.
- Normal flora — mainly coagulase-negative staphylococci and Corynebacterium — occupy niches and inhibit pathogens.
The exception worth memorising: most organisms require an epithelial break to invade, but a small group can penetrate an intact corneal epithelium — Neisseria gonorrhoeae, Corynebacterium diphtheriae, Haemophilus influenzae (some strains) and Listeria. Gonococcal conjunctivitis with a hyperacute purulent discharge is therefore a same-day sight-threatening emergency.
Pathways of disease transmission
The blueprint names "explain pathways of disease transmission." Five routes matter in eye care.
| Route | Mechanism | Ophthalmic examples |
|---|---|---|
| Direct contact | Person to person, skin or mucous membrane | Herpes simplex, adenovirus, gonococcal and chlamydial conjunctivitis |
| Indirect contact (fomites) | Contaminated objects and surfaces | Adenovirus on tonometer prisms, slit lamp chin rests, door handles, multi-dose drop bottles |
| Droplet | Large respiratory droplets travelling a short distance | Haemophilus, Streptococcus, respiratory viruses |
| Airborne | Small particles suspended over distance | Tuberculosis, measles, varicella |
| Bloodborne / percutaneous | Needlestick, sharps, mucous membrane splash | Hepatitis B, hepatitis C, HIV |
| Vertical | Mother to infant during delivery | Ophthalmia neonatorum from gonococcus or chlamydia |
Adenovirus deserves special attention because epidemic keratoconjunctivitis is the classic outbreak in an eye clinic. It:
- Survives on dry surfaces for weeks
- Is resistant to alcohol-based hand rubs — soap and water hand washing is required
- Is shed for around two weeks and the patient is highly contagious for much of that time
- Is readily transferred on tonometer prisms, chin rests, hands and multi-dose bottles
Outbreak control measures: identify suspected cases at reception and separate them; use disposable-tip tonometry or a validated prism disinfection protocol; clean the slit lamp chin rest and headrest after every patient; wash hands with soap and water, not alcohol gel, after contact; use single-use drops rather than multi-dose bottles; and advise the patient about towel separation, hand hygiene and staying away from work or school per local guidance.
Standard (universal) precautions
The governing principle is that every patient's blood and body fluids are treated as potentially infectious, regardless of known diagnosis.
- Hand hygiene before and after every patient contact, before an aseptic task, after body fluid exposure, and after contact with the patient's surroundings. Soap and water when hands are visibly soiled, after caring for a patient with suspected adenovirus, and after glove removal in that setting.
- Gloves for contact with blood, body fluids, mucous membranes, non-intact skin and contaminated items. Gloves do not replace hand hygiene.
- Eye and face protection where splash is possible — irrigation, minor procedures, anterior segment surgery.
- Gowns and aprons where clothing may be contaminated.
- Safe sharps handling — never recap by hand, activate safety devices immediately, dispose at the point of use.
- Respiratory hygiene — cough etiquette, masks where indicated.
- Environmental cleaning — shared equipment disinfected between patients according to a written protocol.
- Safe injection practice — one needle, one syringe, one time; single-dose vials for a single patient.
After an exposure — a needlestick, a sharps injury or a splash to mucous membranes — wash the site immediately, irrigate splashed eyes copiously, report it at once, and attend occupational health for risk assessment and post-exposure prophylaxis. The window for prophylaxis is short, which is why immediate reporting matters more than finishing the clinic.
Why is alcohol-based hand rub inadequate after examining a patient with suspected epidemic keratoconjunctivitis?
Which organism is able to penetrate an intact corneal epithelium?
A Gram stain from a corneal scraping reports Gram-negative rods. Which organism is most consistent with this in a contact lens wearer?
Which tear film components contribute to antimicrobial defence?
A technologist sustains a needlestick injury while assisting with an intravitreal injection. What is the correct immediate action?