BACTERIA

 Bacteria are mass of prokaryotic cell, which means they don't have any true nucleus, instead they have genetic material arranged in a mass called nucleoid. 

CHARACTERSTICS:

• Bacteria are single-celled organisms.
• They lack organelles such as chloroplast and mitochondria.
• Their DNA is double stranded continuous and circular, present in the nucleoid.
• The nucleoid is an irregular shaped region which does not have any nuclear membrane.
• Bacteria also have cell membrane and cell wall which is often made of peptidoglycan.
• Together the cell membrane and cell wall are referred as cell envelope.
• Bacteria mostly reproduce by binary fission, an asexual mode of reproduction.
• Some bacteria exchange genetic material among one another by the process called as horizontal gene transfer. 

SHAPES:

 

• Bacilli are rod shaped bacteria. Bacilli can be solitary or arranged together. Diplobacilli are two bacilli arranged next to each other and streptobacilli are chain of bacilli.
• Cocci are bacteria spherical or ovoid in shape. Some cocci remain attached after binary fission even though two separate cells have been formed. Like, diplococci (two coccus attached together), streptococci (chain of coccus) and staphylococci (cluster of multiple coccus).
• Spiral bacteria are spiral in shape. Spirillum are thick and tough spirals. Spirochetes are spirals that are thin and flexible. Vibrio are comma shaped rod.
• There are other numerous shapes of bacteria like filamentous, square, stalked and star.

CLASSIFICATION:

• Bacteria can be classified in many ways. The first classification scheme was published in 1773 and many more have appeared since.
• Based on several major properties:
• Morphological.
• Anatomical.
• Staining.
• Based on pathogenicity.
• Based on relation between host and organism.
• Nutrition.
• Environmental factors.

     A.     MORPHOLOGICAL

•  Bacteria can be classified according to morphological characteristics like their cell shape, size and structure and their specific arrangement like motility and flagellar arrangement.
• Bacteria can be classified into six major groups on morphological basis.
• 1. TRUE BACTERIA 
• Cocci – These are spherical or oval cells. On the basis of arrangement of individual organisms they can be described as: Monococci (Cocci in singles) – Monococcus spp.,  Diplococci (Cocci in pairs) – Streptococcus pneumoniae, Staphylococci (Cocci in grape-like clusters) – Staphylococcus aureus, Streptococci (Cocci in chains) – Streptococcus pyogenes, Tetrad (Cocci in group of four) - Micrococcus spp., Sarcina (Cocci in group of eight).
• Bacilli – These are rod-shaped bacteria. On the basis of arrangement of organisms, they can be described as: Diplobacilli, Streptobacilli, Palisades, Coccobacilli, Comma-shaped.
• 2. ACTINOMYCETES (actin- ray, mykes - fungus) 
• These are rigid organisms like true bacteria but they resemble fungi in that they exhibit branching and tend to form filaments.
• They are termed such because of their resemblance to sun rays when seen in tissue sections. 
• 3. SPIROCHETES
•  These are relatively longer, slender, nonbranched microorganisms of spiral shape having several coils. 
• 4. MYCOPLASMA
• These bacteria lack in rigid cell wall (cell wall lacking) and are highly pleomorphic and of indefinite shape. 
• They occur in round or oval bodies and in interlacing filaments. 
• 5. RICKETTSIAE and CHLAMYDIAE 
• These are very small, obligate parasites, and at one time were considered closely related to the viruses. Now, these are regarded as bacteria.

       

           B.    ANATOMICAL

• Capsule
•  Capsulate– Streptococcus pneumoniae 
• Non-capsulate – Viridans streptococci
• Flagella 
• Flagellate –  Monotrichous, Lophotrichous, Amphitrichous, Peritrichous
• Aflagellate – Shigella spp.
• Spore 
• Spore-forming – Bacillus spp. 
• Non-sporing – Escherichia coli

        C.   STAINING

• (a) Gram stain: 
• 1) Gram positive: after the gram stain organism which occur violet in colour. 
• 2) Gram negative: Which appear pink or red
•  (b)Acid fast stain: 
• 1) Acid fast organism: after the ziehl – neelsen stain it will show pink in colour. 
• 2) Non acid fast organism: after this stain organism will appear blue in colour.

         D.    PATHOGENECITY

• Pathogens: the organism which is able to spread disease and can cause disease. 
•  Non pathogens: which does not cause and disease; ex: lactobacilli 
• Commensals: normally non pathogenic but sometime they show the disease when immunity impaired

        

          E.   RELATION BETWEEN HOST AND ORGANISM

• saprophytes: free living microbes on dead animals or decaying things that can be found in soil and water and play important role in degradation of organic matter. 
•  Parasites: that establish themselves and multiply in hosts. 
• Commensals: microbes that live in complete live with harmony but they can produce disease when host resistance is lowered. Ex: normal flora organism

         F.     NUTRITION

• Autotrophs 
• Heterotrophs

         G.    ENVIRONMENTAL FACTORS

• Temperature
• Psychrophiles (15-200C) – Pseudomonas fluorescens 
•  Mesophiles (20-400C) – Escherichia coli, Salmonella enterica, Staphylococcus aureus 
•  Thermophiles (50-600C)- Bacillus stearothermophilus 
•  Extremely thermophiles (as high as 2500C)  
• Oxygen Dependence
• Aerobe (grow in ambient temperature, which contains 21% O2 and a small amount of CO2 , 0.03%) 
• Obligate aerobes – Strictly require O2 for their growth (Pseudomonas aeruginosa) 
•  Microaerophilic (grow under reduced O2 , 5- 10% and increased CO2 , 8-10%)- Campylobacter jejuni, Helicobacter pylori    
• pH
• Acidophiles (Lactobacillus acidophilus) 
• Alkaliphiles (Vibrio) 
•  Neutralophiles (pH 6-8) 
•  Majority of the medically important bacteria grow best at neutral or slightly alkaline reaction (pH 7.2-7.6)