4 Major Types of Infections

The following points highlight the four major types of infections. The types are: 1. Infection of Skin 2. Infection of Wounds 3. Infections of Burns 4. Infection of the Eyes.

Type # 1. Infection of Skin:

Clinical Types:

Acne is caused by Propionibacterium acnes, anaerobic diphtherias, coagulase negative staphylococci and micrococci. Toxin of certain organism and free fatty acids may be responsible for acne.

Staphylococcal Infection:

Staph aureus causes boils and carbuncles. These are suppurative lesion, with necrosis at the centre. They discharge a slough with pus. Carbuncle, a large abscess, usually occur in thick collagenous tissue such as back of the neck. Staph, aureus phage group I and II are main causes of boils, carbuncles, styes and sycosis barbae.

Streptococcal Infection:

1. Cellulitis-Superficial infection of subcutaneous tissue.

2. Erysipelas-Spreading infection of dermis.

3. Erythema and Impetigo-Streptococcal impetigo is less common.

4. Scarlet Fever-Usually associated with infection of throat.

Gram-negative infections are usually encountered in moist area of groin and axilla. Coliforms, Bacteroides sp and Proteus sp. May cause abscess between thigh and waist related to a preceding trauma:

Bacteriological Diagnosis:

Specimen: Pus, exudate, swabs from lesions

Culture: Inoculated blood agar is incubated — both aerobically and anaerobically. Identification is done by relevant tests.

Type # 2. Infection of Wounds:

Infection may occur in accidental or post-operative wound due to bacteria:

Pathogenesis, wound infection may be:

(a) Endogenous Infection:

Patient’s own bacterial flora i.e. Staph, aureus, from skin and anterior nares or coliforms.

(b) Exogenous Infection:

Many organisms are exogenous. Staphylococci from skin and anterior nares, organisms from hospital staff and visitors can be spread directly or indirectly by airborne route. Gram-negative enteric bacilli (60%) and Gram-positive cocci (30%) are responsible for hospital acquired infection.

Gram-negative bacilli may be patient’s own flora, or in damp environment. Antibiotic resistant organism (Ps. aeruginosa) from the environment may colonies the patient, traumatized skin (e.g. wound, burn, bedsore).

Laboratory Diagnosis:

Specimen: Pus, exudate and tissue from wound.

Microscopy: Direct Gram-stained smear will reveal the organism.

Culture:

1. Blood agar

2. MacConkey agar for aerobic culture

3. Aminoglycoside agar for anaerobic culture.

4. Robertson s cooked meat medium

Identification:

Isolates can be identified by biochemical and serological tests.

Type # 3. Infections of Burns:

Within 24 hours the large moist exposed surface of burns is colonised by bacteria (e.g. bacterial flora of skin, respiratory tract and intestine, streptococci and/ anaerobic spore bearing bacilli). Staph, aureus is the most common isolate from burn, followed by Ps. aeruginosa and various enterobacteria (e.g., Acinetobacter sp.) and Str. pyogenes group A, B, C and D.

Pathogenesis:

The air-borne organisms reach the burnt area and invade directly the burnt tissue. Laboratory diagnosis is similar to that of wound infection.

Prophylaxis-Local application of antimicrobial agents (silver sulphadiazine cream, silver nitrate solution and chlorhexidine cream) reduce the colonisation of bacteria.

Type # 4. Infection of the Eyes:

Eyelid Infection:

1. Stye:

Staph. aureus causes a small boil or abscess in one of the glands of the lash follicle. They would have originated from anterior nares or implanted from a septic lesion on the body.

2. Infection of lacrimal apparatus:

Infection of glands (dacryoadenitis) occurs from endogenous sources. Dacryoadenitis may be associated with mumps and gonorrhoea.

Conjunctivitis:

Because of flushing action of tears and presence of lysozyme that degrades peptidoglycan of the cell wall of Gram-negative bacilli the conjunctival sac is free from bacteria however, C. xerosis and Staph epidermidis colonies the conjunctival sac.

Conjunctivitis may occur due to conjunctival sac infection by pathogens.

Clinically, it may be acute type, mucopurulent type, non-purulent type and neonatal.

Aetiology:

A. Bacterial Agents:

1. Staph, aureus causes acute conjunctivitis

2. H. influenza

3. H. aegypticus

4. Moraxella lacunata

5. Chlamydia trachomatis

6. Str. pneumonia

In newborns, conjunctivitis caused by Staph, aureus is commonly observed in “Sticky eyes“. In older infants and children, infection due to H. influenza and Str. pneumonia is more common. C. Trachomatis (Serovars D-K) causes inclusion conjunctivitis and trachoma (A, B, C). Endemic area, trachoma is very common amongst children and is a very common cause of blindness in the world.

B. Viral Agents:

1. Adeno viruses (types 3, 7, 8, and 14)

2. Herpes viruses (HSV1)

Keratoconjunctivitis is caused by Adeno viruses type 7, 8 and Herpes virus (HSV1).

Neonatal Conjunctivitis:

Conjunctivitis in neonates may be acquired either during birth from the mother’s genital tract or from some other external sources within 3 weeks of birth.

Aetiology:

(a) From the genital tract-Infection of eye acquired by the newborn during birth is called “Ophthalmia neonatorum“:

1. N. gonorrhoeae

2. C. trachomatis (D-K serovars)

3. Group B haemolytic streptococci.

(b) From the external source-Staph. aureus causes sticky eyes in the newborn.

Keratitis:

It is inflammation of the cornea. The infective organisms of conjunctivitis can spread deeper into cornea leading to ulcers or a spreading keratitis.

Causes:

1. Herpes virus

2. Foreign body with secondary infection due to bacteria or fungi.

Mycotic Keratitis:

Corneal infection by fungus occur secondarily following injury. Saprophytic fungi, Aspergillus and Fusarium species are the common causes of infection by opportunistic fungi.

Laboratory Diagnosis:

A. Bacterial infection except Chlamydial:

Specimen:

Exudate collected by a sterile platinum loop directly from the patient’s eye. Preparation of smear and culture should be done at the bedside. In case of delay in transport, swab of exudate should be placed in Stuart’s transport medium and despatched to the laboratory.

Microscopy:

Gram:

Stained smear reveal the presence of causative organisms.

Culture:

Material is cultured on blood agar, and chocolate agar, incubated at 37°C in an atmosphere of 5-10% CO₂.

Identification:

Organism are identified by biochemical test and serotyping.

B. Chlamydial infection:

Specimen:

Scrapings from conjunctiva. Microscopy-Film is examined by immunofluorescence.

Culture:

Tissue culture is performed in cycloheximide treated or irradiated McCoy cells for 2-3 days.

Identification:

The organism produce intracytoplasmic inclusion which are identified by Giemsa stain or immunofluorescence.

C. Mycotic Infections:

Specimen:

Scrapings from the base or edge of corneal ulcer.

Microscopy:

1. In 10% KOH mount, the fungi appear as branched, septate hyphae

2. Methylamine silver or PAS staining shows filamentous fungi with hyphae.

Culture:

Scraping material is inoculated on Sabouraud’s dextrose agar.

Orbital Cellulitis:

It is serious Infection of the cellular tissue of the orbit. It may usually result from spread of infection from adjacent tissue (teeth and sinuses). Complications are panophthalmitis, brain abscess, meningitis. Painful swelling and protrusion of the eyeball may occur.

Aetiology:

Staph, aureus and other pyogenic organisms can cause orbital cellulitis.

Laboratory Diagnosis:

Specimen of pus collected by surgical incision can be subjected to the laboratory examination which is similar to that of conjunctivitis.

Panophthalmitis:

It is an inflammation of whole substance of the eye derived from infections of the outer eyes, foreign bodies and penetrating wounds (traumatic or surgical).

Aetiology:

Staph, aureus

Gram negative enteric bacteria (Ps. aeruginosa, Proteus sp., Klebsiella sp., E. coli, Enterobacter sp.)

Diagnosis-Pus aspirated from the anterior chamber of the eye should be cultured.

Choroiditis and Chorioretinitis:

Aetiology: Bacterial, viral, fungal, protozoal infections may cause granulomatous lesion of choroid and retina.

Bacteria: M. tuberculosis, Br. abortus, Tr. pallidum.

Virus: Congenital rubella, CMV

Fungus: C. albicans

Protozoa: Toxoplasma gondii

Helminths. Toxocara canis causes retinitis.

Diagnosis can be done by skin and diagnostic test.