Microorganism are very small microscopic structures that are capable of free living. Some of the the microorganisms are non-pathogenic and live on the body of human beings i.e on the skin, in the nostrils, in the intestinal tract etc and they are called COMMENSALS. The organism that are capable of causing disease are called pathogenic organisms. There are two groups depending upon the structure of cells
1-Prokaryotic
2-Eukaryotic
Prokaryotes: This group includes those organisms that have a very simple cell structure and nuclear material in the form of single chromosome but is not surrounded by nuclear membrane. They devide by simple binary fusion. Examples are bacteria, Mycoplasma, chlamydia and rickettsiae.
Eukarotes: These organisms have complete cell structure similar to higher organisms. The nuclear material is bounded by a nuclear membrane to form a nucleus. They have more then one chromosome,complete enzymr system of their own and devide by mitosis
Examples are fungi and protozoa
Classification Of Bacteria:
Bacteria can be classified depending upon:
1-Morphology
2-Gram staining
3-Requiment of Oxygen
DNA homology
MORPHOLOGICAL CLASSIFICATION:
On the basis of morphology bacteria are divided into following groups:
A- COCCI: round or oval in shape
B- BACILLI: rod shaped
C- VIBRIOS: comma shaped
D- SPHIROCHAETED: spiral like
Intermediate shaped like cocco-bacilli also exist
CLASSIFICATION BASED ON GRAM STAINING:
Bacteria are divided into Gram-negative and Gram-positive on the basis of their cell wall structure
a- Gram-positive: Bacteria staining purple in Gram stained smear. They have thick layer of peptidoglycan
b- Gram-negative: Bacteria staining pink in Gram stained smear. Gram-postive bacteria when dead may stain red. They have thick outer membrane
c- Gram-variable: The organism is Gram-positive but appears Gram-negative or is Gram-negativr but appears Gram-positive
CLASSIFICATION BASED ON OXYGEN REQUIREMENT:
a- Strict Aerobes: These do not griw in absence of oxygen
b- Anaerobe: These can be of two types
1). Strict (obligatory) anaerobes:
Bacteria that can grow only in absence of oxygen
2) Facultative anaerobes: These can grow both in presence or absence of oxygen. Most of the commonly isolated bacteria belong to this group
c- Carboxyphilic: These require presence of high percentage(10%) of carbon dioxide
d- Microaerophilic: These require only small amount of oxygen for their growth and higher concentration of oxygen will kill the organism
Classification On Temperature Requirement:
Based on temperature requirement for their growth bacteria are classifieds into following three groups
1- Mesophilic
2- Psychrophilic
3-Thermophilic
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