MICROBIOLOGY

BY: SUMAYYA NASEEMOptometrist

School Eye Health Programme, PakistanCHEF International

Introduction to Microbiology

The agents of human infectious diseases The agents of human infectious diseases belong to 5 major groups of organisms:belong to 5 major groups of organisms:

•BacteriaBacteria•VirusesViruses•FungiFungi•ProtozoaProtozoa•HelminthesHelminthes

• Out of these bacteria , fungi & protozoa are unicellular.

• Some of the fungi are also relatively simple multi cellular.

• The helminthes are complex multi cellular and together with the protozoa are commonly called parasites.

• Viruses are different from the rest as they are not cells but can replicate only with in cells.

• Collectively all of them are called micro organisms.

• The organisms which are capable of causing disease are called pathogenic organisms.

• Some of the micro organisms are non. Pathogenic and live on the body of human beings and are called commensals.

A. BACTERIA:

Bacteria can be classified depending upon:-

• Morphology • Gram Staining• Oxygen Requirement

MORPHOLOGICAL CLASSIFICATION

Bacteria are classified by shape into three basic groups:-

• Cocci :- Round or Oval• Bacilli :- Rod Shape• Spirochetes :- Spiral like• Intermediate shapes like cocco-

bacilli may exist

In addition to their characteristic shapes, the arrangement of bacteria is important e.g

• Cocci occurring in pairs called diplococci

• Cocci occurring in chains e.g. Streptococci

• Cocci occurring in grape like clusters e.g. Staphylococci

CLASSIFICATION BASED ON GRAM STAINING

• Because bacteria are colorless and usually invisible to light microscopy.Colorful stains have been developed to visualize them.

• The most commonly used is the Gram stain which separates the organisms into:-

• GRAM +ve :- When bacteria stain blue in Gram stained smear,e.g clostridia,C.B.D, streptococcus, staphlococcus

• GRAM –ve :- When bacteria stain red in Gram stained smear ,e.g. N.gonarrhea, E.coli, Pseudomonas

• The difference is in the cell walls of GM +ve & GM –ve bacteria.

CLASSIFICATION ACCORDING TO O2 REQUIRMENT

STRICT AEROBES• They do not grow in the absence of oxygen but

they grow in the presence of oxygen e.g. E.coli

ANAEROBES• They grow best in absence of oxygen e.g.

clostridia

MICRO-AEROPPHILIC• They require small amount of oxygen

Intermediate characters like facultative anaerobes are also found

NORMAL FLORA

The normal flora comprises a permanent population of organisms that are normally present in the following sites :-

• Skin e.g. staphylococci• Respiratory tract e.g. Streptococci• Intestinal tract e.g. E.Coli, Bacteroides• Genitourinary tract e.g. Lactobacilli

The bacteria which are capable of causing disease are called pathogenic, many but not all, infections are communicable i.e. are spread from person to person

BACTERIAL INFECTIONS OF THE EYE

In the eye bacteria can cause the following diseases:-

1. Bacterial Conjunctivitis2. Bacterial Keratitis3. Bacterial Uveitis4. Optic Neuritis5. Stye

1. BACTERIAL CONJUNCTIVITIS

Many different bacteria can cause conjunctivitis e.g.

– STAPHYLOCOCCI can cause acute conjunctivitis.

– HAEMOPHILUS INFLUENZA causes seasonal epidemics in hot dusty climate.

– Pneumococcus , Meningococcus and Streptococcus cause acute conjunctivitis

– Trachoma caused by Chlamydia trachomatis is the most common preventable cause of blindness through out the world. It causes two patterns of diseases in the eye, The severe form is trachoma & The milder form is inclusion conjunctivitis.

― Granulomatous conjunctivitis : this is a rare form caused by the organisms causing:-

• TB• Syphilis

Bacterial Conjunctivitis

Trachoma

PREDISPOSING FACTORS FOR

BACTERIAL CONJUNCTIVITIS

• All the types of bacterial conjunctivitis are more common and more serious in hot climates for the following reasons:-

• Warm & humid climates• Dust and solar radiations which irritate the

conjunctiva • Flies which carries infections from eye to eye• Over crowding• Poor ventilation• Poor hygiene

2. BACTERIAL KERATITIS

Predisposing factors are:-• Following injury• In an unhealthy cornea• Hot dry air or sand particles in

atmosphere • Vit-A deficiency can also lead to

primary & secondary Keratitis

• Bacteria causing Keratitis are almost the same as those causing conjunctivitis.

• This is because the epithelium of the cornea is directly continuous with the epithelium of the conjunctiva.

• A corneal ulcer develops if there has been a loss of some of the epithelium due to the infection.

Bacterial Ulcer

3. BACTERIAL UVEITIS

Bacteria causing uveitis are:-

– Mycobacterion TB– Mycobacterium Laprae– Syphilis

Cell & Flare

4. OPTIC NEURITIS

Bacterial infection can damage the optic nerve in:-

• Typhoid• Syphilis

Optic Neuritis

5. STYE

• STYE is an infection of eyelash follicle by staphylococci

Stye

B. VIRUSES

Viruses are acellular microorganisms with the following properties:-

• The viruses have an internal core composed either of DNA or RNA.

• Viruses must produce with in the cells.• They do not undergo binary fission.• Some viruses have an outer protein coat

called envelope.• The size of largest virus i.e Pox virus is

equal to the smallest bacteria i.e Chalamydia.

Virus

CLASSIFICATION

Classification according to the nuclear material

• RNA VIRUSES e.g. HIV, Polio virus• DNA VIRUSES e.g. Herpes, CMV,

Adenovirus etc

VIRAL INFECTIONS OF THE EYE

1. Viral Conjunctivitis2. Viral Keratitis3. Uveitis4. Optic Neuritis5. Herpes Zoster Opthalmicus

1. VIRAL CONJUNCTIVITIS

Unlike bacteria, the viruses live inside the cells and are therefore immune to antibiotics.

There are following types of viral conjunctivitis:

– ANDENOVIRUS Conjunctivitis– MEASLES Conjunctivitis– MOLLUSCUM CONTAGIOSUM– HERPES SIMPLEX Conjunctivitis

There are typical sign and symptoms of viral conjunctivitis i.e.

• The secretions are watery• There is a gritty sensation in eye• The blood vessels of conjunctiva are

dilated• Conjunctival scrapping shows

lymphocytes and a few monocytes

Viral Conjunctivits

2. KERATITIS/CORNEAL ULCER

There are many different types of corneal ulcers. But they have typical signs and symptoms i.e. :-

• Pain• Blurred vision• Photophobia• Watering

Different type of viruses can lead to ulcer formation in a diseased cornea but the common type of virus causing corneal ulcer in an otherwise healthy eye is Herpes Simplex. It spreads from person to person.

Dendritic Ulcer

3. UVEITIS3. UVEITIS

Viral causes of uveitis include:-Viral causes of uveitis include:-

•Herpes SimplexHerpes Simplex•Herpes ZosterHerpes Zoster•Cytomegalo virusCytomegalo virus

Uveitis

4. OPTIC NEURITIS

Viruses can damage the optic nerve especially in a debilitated / Malnourished patient. The important one is :

• Measles

5. OPHTHALMIC HERPES ZOSTER

• This is also called shingles and is caused by Herpes Zoster / varicella virus which also causes chicken pox

• The virus infection only involves one sensory nerve on one side of the body

• Herpes Zoster usually occurs in the elderly or immuno suppressed patients

HZO

C. FUNGUS

• The study of fungi is called Mycology, and the diseases they cause are called Mycoses.

• There are two types of fungi :-(i) YEASTS

– Yeasts are unicellular– They rarely form filaments– They grow by asexual budding

(ii) MOLDS• Molds are multicultural • They grow as long filaments• These filaments are called HYPHAE• The network formed by filament is

MYCELIUM

FUNGAL INFECTIONS OF EYE

1. Conjunctivitis2. Keratitis3. Uveitis

1.CONJUNCTIVITIS

• Fungi do not cause acute conjunctivitis, it rather causes a granulomatous inflammation with regional lympodenopthy e.g. as in sporotrichosis

2. KERATITIS/ CORNEAL ULCERS

• Fungal infections of the cornea are common in the hot humid climate of tropics

• Fungi usually live on plant matter; abrasions to the cornea from twigs, thorns or seeds have a greater risk of fungal contamination

• Aspergillus and • Candida

Aspergillus niger & Candida albicans

Aspergillus Keratitis

3. UVEITIS

Fungi Causing Uveitis Are:

• Candida • Histopasma

Iritis

D. PROTOZOA

• Protozoa are unicellular parasites in turn is a living organism which has adopted itself to existence in or on another animal called host.

• The protozoa can eat, respire and reproduce without help.

PROTOZOA AFFECTING THE EYE

1. ACANTHAMOEBA• Acanthamoeba is a wide spread protozoan

organism found in most tap water.• It can invade the cornea to produce a

typical chronic central ulcer.• Usually affect soft contact lens wearers.

Keratitis due to Acanthamoeba

2. TOXOPLASMA • Toxoplasmosis is a common & important

cause of chorioretinitis.• The protozoan is wide spread throughout

the world.• The main host in the life cycle of

toxoplosma is cat.• Cysts are found in cat’s faeces.

• The secondary host is usually man or some other animal & if the cysts are eaten they are changed into active infective form.

• They invade the tissues & blood producing a generalized illness with lymphadenopathy, then again forming tiny cysts.

• To complete the life cycle the cat becomes infected by eating meat containing these cysts.

• If a pregnant woman become infected, then the organism cross the placenta & infect the fetus

• In the fetus they infect the CNS & Retina leading to inflammation in the retina & choroids i.e. chorioretinitis

PARASITES AFFECTING THE EYE

• Infections by parasites are one of the features of tropical diseases & there is a vast range of parasites which can infect the body. Important features of parasitic diseases of the eye are:-

• Most are spread by insect vector• The warm climate encourages the growth & survival of

the parasites, their larvae & their eggs out side the human body

• Poor sanitations & poor water supply• People living in close contact with domestic animals• Poverty & poor cooking facilities

• Some of these parasites have complex life cycles, living in two or even three different hosts to complete their life cycle

Important Parasitic Diseases

Some of the important parasitic diseases are:-

• Onchocerciasis• Loiasis• Toxocara• Cysticercosis• Hydatid disease

1. ONCHOCERCIASIS

• This is a parasitic disease caused by Onchocerca volvulus which is a filarial worm.

• Large number of larval microfilaria are found in the skin

and eyes.

• Onchocerciasis is spread by the bite of black fly.

• This fly breeds in the fast flowing rivers. The disease is commonly called river blindness.

• About a quarter million people are blind and half a million partially sighted from this disease.

Sclerosing keratitis in Onchocerciasis

2. LOIASIS

• It is also caused by filarial worm, carried by fly.

• The life cycle is similar to O. volvulus except that it is the adult worm which causes the symptoms & there is no threat to the sight

• The site of bite is called CALABAR swelling

Loa Loa Worm under the Conjunctiva

Calabar Swelling

3. TOXOCARA

• Toxocara is a nematode living in the intestine of dog.

• The larva is called visceral larva migrans leads to a generalized Ant / post uveitis.

4. CYSTICERCOSIS

• It is caused by pork tape worm i.e. tenia solium.

• They from orbital & into ocular cysts.

5. HYDATID DISEASE

• Hydatid disease is caused by dog tapeworm echinococcus granulosus most common site of infection is liver but may enter the orbit.

• Rupture of cyst can lead to Anaphylaxis.