4.5 AZOBACTER AND RHIZOBIUM

Introduction Both are soil bacteria Azobacter and Rhizobiaceae bacterial

species are nitrogen fixers Azobacter is a free-living bacteria Rhizobium basically in soil Most bacteria in soil are about one micron

in length or diameter. Their size varies with their environment. Bacteria in -high levels of nutrients ,larger

than those in nutrient- poor .

Azobacter

Azobacter - Soil bacteria The majority of bacteria in soil usually

occur as single cells. Bacteria sometimes join together in

chains or clusters. They mainly have one of two shapes - spheres (called cocci) and rods (called bacilli).

Other bacteria have more varied shapes including spirals and long thin hyphae (although these are less common).

Forms of Nitrogen Urea CO(NH2)2 Ammonia NH3 (gaseous) Ammonium NH4 Nitrate NO3 Nitrite NO2 Atmospheric Dinitrogen N2 Organic N

Roles of Nitrogen Plants and bacteria use nitrogen in the

form of NH4+ or NO3

-

It serves as an electron acceptor in anaerobic environment

Nitrogen is often the most limiting nutrient in soil and water.

Nitrogen is a key element for

amino acids nucleic acids (purine, pyrimidine) cell wall components of bacteria (NAM).

As nitrogen fixer These bacteria take nitrogen from the air

(which plants cannot use) and convert it into a form of nitrogen called ammonium (NH4

+), which plants can use. The nitrogenase enzyme controls the

process, called nitrogen fixation, and these bacteria are often called "nitrogen fixers".

It converts atmospheric nitrogen into ammonium, but in the condition of :

An easily degradable carbon source is available

Any nitrogen compounds such as ammonium or nitrate, are not already present in substantial concentrations

Soil pH levels are between 6 and 9 High levels of phosphorus are

present Very low levels of oxygen are

present

Benefits as biofertilizer These are primarily used to enhance the

growth of several species of plants and crops.

A correct proportion of bacterial growth could ensure a high quality of biofertilizer

Its capability in maintaining the pH, safe to use and compatible as chemical fertilizer

Rhizobium Rhizobium is primarily a gram negative,

motile, non-sporulating rod that requires a plant host.

Species under this genus:1. R. phaseoli2. R. leguminosarum3. R. trifolii

Other genus are cowpeas Rhizobium and Bradyrhizobium

In agricultural Rhizobia belong to a specific group of bacteria

that form a mutually beneficial association, or symbiosis, with legume plants.

Rhizobia are found in soils of many natural ecosystems. They may also be present in agricultural areas where they are associated with both crop legumes (like soybean) and pasture legumes (like clover).

Usually, the rhizobia in agricultural areas have been introduced at sowing by applying an inoculum to the exterior of the seeds as liquid formations or pellets.

Rhizobium

4.6 ENTEROBACTERIACEA

EEnteric Gram negative

aerobes rods

Sub-grouping LACTOSE FERMENTING(PINK PIGMENT IN MAC AGAR)

NON-LACTOSE FERMENTING(NO PINK PIGMENT IN MAC AGAR)

1. ESCHERICHIA2. KLEBSIELLA3. ENTEROBACTER4. CITROBACTER

1. SALMONELLA2. SHIGELLA3. PROTEUS4. MORGANELLA5. PROVIDENCIA6. SERRATIA

Morphology Gram negative Non-sporing Rods 1-4 X 0.6µm

Motility Mostly surrounded by flagella

(peritrichous) and are motile Non-motile enterobacteriaceae are

Shigella and Klebsiella In Escherichia and Morganella, most of

the strain are motile, but some are non-motile

Special case like Yersinia, contain species that are motile at 25oC but non-motile at 35 – 37oC

KLEBSIELLA E.COLI

Culture Most will grow in wide temperature range in

ordinary culture media including NA and BA Selective media is required to recover

Salmonella and Shigella On BA, enterobacteria produce large, shiny,

grey colonies that may be hemolytic Species that produce hydrogen sulphide

often show a green colour around the subsurface colonies (Klebsiella) and capsulated strains (Escherichia) produce large mucoid colonies.

E.coliIN NUTRIENT AGAR IN MACCONKEY AGAR

Klebsiella spIN MACCONKEY AGAR IN BLOOD AGAR

Salmonella spIN SS AGAR IN XLD AGAR

Toxin production Exotoxin (enterotoxin) is produced by

Shigella dysenteriae and toxigenic strain of Escherichia coli (ETEC)

When lysed, enterobacteria will release endotoxin from their cell wall

*The feature above is applied to all Gram negative rods

Serology There are three antigens possesed by some or all

enterobacteria 1. O, or somatic antigens, found in the wall of bacterial

cell. These antigen are used to subgroup enterobacteria such as Salmonella and Escherichia sp. O antigen is heat-stable.

2. K, or surface antigen, found surround the cell wall and can therefore interfere with O antigen grouping. They can be inactivated, however, by heat. Eg. Vi antigen for Klebsiella and B antigen for strain of E.coli

3. H, or flagella antigen, posesses by motile enterobacteria, they are found in the flagella and are heat-labile. H antigens are used to identify Salmonella serovars within their somatic groups

Antigenic structure of Enterobacteriacea

TASKS DISCUSS 5 GENUS OF

ENTEROBACTERIACEAE INCLUDE –MORPHOLOGY,

BIOCHEMICAL CHARACTERISTIC, IMPORTANCE OF THE CULTURE.

The end