YersiniaPasteurellaFrancisella

INTRODUCTIONThe name Yersinia is given after

Alexander Yersin who discovered the plague bacillus.

The genus Yersinia belongs to the family Enterobacteriaceae.

The genus Yersinia contains three medically important species

1.Y.pestis

2.Y.pseudotuberculosis 3.Y.enterocolitica

The genus pasteurella contains several related bacteria causing hemorrhagic septicaemia in different species of animals and occasionally producing human infections.

The genus francisella consisting of F.tularensis is named after Francis for his contribution on tularaemia,caused by this bacillus.

YERSINIA PESTISCausative agent of plague formerly

known as pasteurella pestis was isolated independently and simultaneously by Yersin and Kitasato

MORPHOLOGY short ovoid,gram negative bacillus,about 1.5×0.7microns in size,with rounded ends and convex sides,occur singly,in short chains or in small groups.

When stained with methylene blue it shows bipolar staining (safety pin appearance)with two ends darkly stained and central area clear.

Pleomorphism is very common.it is characteristically enhanced in 3%Nacl.

In old cultures involution forms –coccoid,club shaped,filamentous and giant forms are observed.

The bacterial body is surrounded by a slime layer (envelope or capsule).

It is non motile,non sporing and non acid fast.

Morphology-safety pin appearance

CultureNutrient AgarColonies are

small,delicate,transparent after 24-48 hrs of incubation.

MacConkey Agar Colourless colonies are

formed.

Blood agarColonies are non haemolytic and dark brown due to absorption of haemin pigment.

Broth In nutrient broth,a flocculent growth

occurs at bottom and along sides of tube.

Ghee broth A characteristic growth occurs which

hangs down from surface resembling stalactites(Stalactite growth).

Stalactite growth on ghee broth

Biochemical reactions Y.pestis ferments glucose,mannitol and

maltose with production of acid but no gas.

Lactose and sucrose are not fermented. Catalase-positive Urease-negative Indole-negative MR-positive Vp-negative Citrate-negative

Biochemical reactions

Urease

Indole

MR

catalase

On the basis of fermentation of glycerol and redution of nitrate,Devignat distinguished 3 physiological varieties.

This is of epidemiological significance because of different geographical distribution.

Y.pestis var. orientalis

Y.Pestis var. antiqua

Y.Pestis var. mediaevalis

RESISTANCEIt is destroyed by heat at 55˚c

in 15 min and also by sunlight,drying and 0.5% phenol in 15 min.

Survive for several months in soil of rodent burrows.

Remains viable for long periods in cold and moist environments.

ANTIGENIC STRUCTUREThese are antigenically homogenous

and serotypes do not exist.The antigenic structure is complex

and about 20 diff antigens have been identified.They include :

A heat labile protien envelope antigen known as fraction-I,best formed in cultures incubated at 37˚c.

Cont…….Fraction-I inhibits phagocytosis.It is present only in virulent strains.It has imp role in stimulating

protective immunity in mice and man.

V and W proteins which are also formed by virulent strains have been considered to be virulence factors as they inhibit phagocytosis.

Cont… Virulent strains produce-

1.bacteriocin(pesticin 1) 2.fibrinolysin 3.coagulase. Pesticin 1 inhibits strains 1.Y.pseudotuberculosis 2.Y.enterocolitica 3.E.coli

Plague toxins-refers to atleast 2 classes of toxins found in culture filtrates.

First is the endotoxin,a liposaccharide similar to endotoxin of enteric bacilli.

Second possess properties of both exotoxins and endotoxins.

Also called murine toxins as they are active in mice and rats but not in guinea pigs,rabbits and primates.

On injection,into exp animals these produce local edema and necrosis with systemic effects on peripheral vascular sys. and liver.

Virulence also appears to be associated with an unidentified surface component which absorbs haemin and basic aromatic dyes in culture media to form coloured colonies.

Virulence has also been associated with the ability to synthesize purines.

PathogenesisY.pestis is a natural pathogen of

rodents and causes zoonotic disease called plague.

It is transmitted to man via the bite of infected rat flea.(X.cheopis).

The incubation period is 2-6 days.Two natural cycles of plague exist 1.URBAN PLAGUE 2.WILD OR SYLVATIC

PLAGUE.

In human beings,plague occurs in 3 forms .

1.Bubonic plague 2.Pneumonic plague 3.septicemic plague.

BUBONIC PLAGUEAfter an incubation period of 2-

5 days,lymph nodes infected.As bacillus usually enters

through bite on legs,inguinal nodes are involved.

Hence the name ‘bubonic’.

The glands become enlarged and suppurate.

Patient develops fever,chills and malaise.

Often bacillus reaches blood stream and gets widely disseminated leading to gangrene of skin,fingers and penis.

gangrene

PNEUMONIC PLAGUEHighly infectious form of

plague involving the lungs producing haemorrhagic pneumonia.

Transmission-droplet infection.Cyanosis is very prominent.

SEPTICEMIC PLAGUEThe presence of bacteria in blood.Usually the terminal event in

bubonic or pneumonic plague.Massive involvement of blood

vessels results in haemorrhages in skin and mucosa.so, the disease is given the name ‘BLACK DEATH’.