Pathogenic Bacteria
These bacteria cause great losses to plant and animal population by causing several diseases.
Reduction of Soil Fertility
Some facultative anaerobic bacteria are available mostly in the oxygen deficient soil which reverse the nitrifying process, thereby causing the loss of a part of its combined nitrogen. They break down the nitrates in a stepwise manner either to ammonia through assimilatory nitrate reduction or to N2 through dissimilatory nitrate reduction.
Spoilage of Food
In favourable temperature and humidity, bacteria can grow luxuriantly in many food materials. They change the flavour, appearance and smell of food.
Different species of Streptococcus, Micrococcus and Lactobacillus cause spoilage of milk and different milk products. The exotoxin produced by Clostridium botulinum causes botulism disease showing the symptoms like swelling of tongue, double vision and respiratory disturbances.
Pollution of Water
Some bacteria pollute water and make it unsuitable for drinking. Diseases like cholera (Vibrio cholerae), typhoid fever (Salmonella typhi) and bacillus dysentery (Shigella dysenteriae) are commonly transmitted by drinking water.
Biological Warfare
Bacteria causing diseases like anthrax, black-leg, tuberculosis, etc., are used as secret war-agents.