Download Insects and Microbes

Insect Pathology
Prof. Abdelwahab A. Ibrahim
Entomology Department, Faculty of Science, Benha University
Prof. Abdelwahab A. Ibrahim
BACTERIA
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Bacteria are prokaryotes, They have a cell wall. no well-defined nucleus or organelles.
Bacterial pathogens are classified according to many factors such as infective dose, site of
infection, host range and mode of action.
Insect bacterial pathogens includes two main groups
Spore formers and non-spore formers.
Spore forming bacteria which may be:
Obligate:
- Grow and divide only inside the insect host as:
- e.g. Bacillus popilliae and B. lentimorbus (causing milky disease in Japanese
beetles)
- e.g. Clostridium malacosomate and C. brevifaciens.
Facultative:
- Can grow easily in Petri dishes or in liquid culture but need specific conditions.
for spore formation in artificial media.
- It can be divided into 2 groups:
- Crystalliferous : It can form protein crystals toxic to insect larvae as B. thuringiensis
- Non- Crystalliferous: It form spores but no crystals as B. cereus.
NonSpore forming bacteria which may be:
- Facultative as Serratia marcescens
- Potential pathogens as Pseudomonas, Aerobacter, and Proteus.
Prof. Abdelwahab A. Ibrahim
BACTERIA
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The most common bacterial flora found in digestive tracts of
healthy insects are gram-negative small rods, resemble the flora
of higher animals. Many of these bacteria are saprophytes;
others are obligate anaerobes..
 The bacteria, in different insect species, may range from very
small to rods to large spirochete-like forms. Some of these
bacteria are intimately attached to the midgut epithelium and
chitinous wall of the hindgut. Adult insects pass some bacteria
from generation to generation in association with the eggs.
Diseases caused by some spore forming bacteria:
 The most important bacteria causing diseases are the spore
forming bacteria which have dormant stage resisting
environmental conditions outside the host.
 Family Bacillaceae contains two important genus causing insect
diseases:
Genus Bacillus (aerobic or facultative aerobics).
Genus Clostridium (anaerobic or aero-tolerant).
Prof. Abdelwahab A. Ibrahim
BACTERIA
 Pathogens of Genus Bacillus
 Bacteria of genus Bacillus are
able to form endospores and
can secrete enzymes that
analyze the cell walls of host
during growth (exo-enzymes)
 They are aerobic, unicellular,
usually
bacilliform,
spore
forming bacteria.
 Infection occurs after ingestion
of bacterial cells or spores.
They
mainly
affect
phytophagous larval stages.
 Bacillus spp can be easily
produced by fermentation.
P. japonica) larvae
Prof. Abdelwahab A. Ibrahim
BACTERIA
 Bacillus thuringiensis
 During sporulation this bacteria produces
a large proteinaceous crystal (parasporal
body or ẟ- endotoxins) which is
bipyramidal in shape and also a thick
walled endospore. The crystal is an
endotoxin which dissolves inside the host
in alkaline gut fluids and releases toxic
polypeptides.
 This genus was described by Berliner
1911, and contains several serotypes,
most of them are pathogenic to
Lepidoptera larvae.
 B. thuringiensis serotype H-14 is highly
pathogenic to mosquito larvae.
 Most vegetative cells of this genus are
not pathogenic, and most toxic effects
are largely due to the crystals produced
during sporulation.
P. japonica) larvae
Prof. Abdelwahab A. Ibrahim
BACTERIA
 Bacillus thuringiensis
 When B.t. is grown on suitable media it
produces heat stable ß-exotoxin in the
medium during vegetative growth and
before sporulation. This exotoxine affects
insects manly during metamorphosis and
cause the appearance of malformed
insects or prevents pupation. It mainly
inhibits polymerization of RNA.
 The sensitivity of insects to infection
depends on many factors as age,
temperature, humidity, food source, etc..
 Insects infected with B.t. stop feeding,
became paralyzed in a short time, in
addition to a decrease in gut alkalinity
and an increase in hemolymph alkalinity
which cause septicemia and death.
 All members of this group are quite safe
to non target organisms especially
P. japonica) larvae
Prof. Abdelwahab A. Ibrahim
BACTERIA
 Pathogens of Genus Bacillus
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B. sphaericus:
Certain strains produce a proteinaceous toxin which poisons
mosquito larvae. Host death may also be induced by the spores
alone.
B. popilliae:
Produces no toxins in the infection cycle. Japanese beetle (Popillia
japonica) larvae ingest spores in the soil which germinate and
produce vegetative cells which fill the gut in three to five days. Some
cells penetrate the gut wall, then grow and sporulate in the
haemolymph. 14-21 days after initial infection the insect body is
swollen and creamy white (milky disease). After death, the spores are
released into the soil and establish persistent infection sites.
B. cereus:
B. cereus is a Saprophytic non spore forming bacteria.
When insect larvae feed on this bacteria, the body relaxes, and brown
spots appear on the skin, then larvae stop motion and the body is
covered with brown color. After death the body becomes soft and
black , internal body organs crash and sticky exudate appears on the
surface. The ability of bacteria to cause disease is largely due to its
secretion of phospholipase c enzyme. Similar symptoms were
observed When healthy larvae were exposed to the enzyme . It was
also found that acidic PH in the gut is necessary for infection.
Spore of B popilliae
P. japonica) larvae
Prof. Abdelwahab A. Ibrahim