Part I General Microbiology

Microbiology is the science that is primarily What is Microbiology? Microbiology is the science that is primarily concerned with the study of microorganisms that are too small to be seen with the unaided human eye. Medical Microbiology is the study of the causes and management of infectious diseases. These can be caused by viruses, bacteria, microfungi and protozoa.

Microorganisms are divided into two groups, based on the presence or absence of a nuclear membrane surrounding the nuclear material. Prokaryotes: (pro: primitive and karyon: nucleus): E.g.: Bacteria and rickettsiae. Eukaryotes: (Eu: true and karyon: nucleus) E.g.: fungi, protozoa, human and animal cells.

Bacteria: Bacteria are generally simple structures Bacteria: Bacteria are generally simple structures. The bacterial cell lacks a membrane-bound nucleus. Because of this, bacteria are described as prokaryotes. It is measured by µm(1/1000mm).it's size is about 0.5-10 µm.

Viruses: obligate intracellular parasites Fungi: All fungi are eukaryotic ,vary in size from 5 to100 µm. Viruses: obligate intracellular parasites incapable of independent existence .Their sizes vary from 20 to 400 nm(1/1000 µm).

1 2 3 4 5 6 7 8 9 10 11 12 Eukaryotes Prokaryotes 5 – 100 µm Average cell size 1 Absent except fungi & algae Present except mycoplasma Cell Wall 2 Sterols present Sterols Absent (except in mycoplasma). Cytoplasmic membrane 3 Present; Absent Nuclear membrane 4 Present Nucleoli 5 Multiple. Single Chromosome 6 Mitochondria 7 Complex Simple Enzyme system 8 Sedimentation constant 80S. Sedimentation constant 70 S Ribosome 9 Mitosis. Binary fission Multiplication 10 Involves mitochondria Involves cytoplasmic membrane Respiration 11 Fungi, algae, protozoa, higher plants & animals. Bacteria, Rickettsia. Examples 12

BACTERIAL CELL STRUCTURE: The morphological description of bacterial cell includes: size, shape, arrangement, motility and staining properties Size: Bacteria are unicellular organisms measured in micrometer (µm=0.001 mm = 0.000001 meter)

SHAPE &ARRANGEMENT:: Cocci: These are spherical in shape, may be arranged in pairs (diplococcic), in chains of variable length e.g. streptococci, in fours (tetrads) or in groups (e.g. staphylococci) Bacilli: These are rod shaped bacteria which may be arranged in pairs e.g. TB, in angles e.g. diphtheria, or in groups e.g. Leprosy Spirals: spirella(rigid),and spirochetes (flexible). Pleomorphic: variable in shape.

Staining: Bacteria can be stained by :- a) Simple stain :e.g. methylene blue, crystal violet or red fuchsin. Cells and structures stained with them give the same colour to the film. This stain reveals the characteristic shape ,size and arrangement.

one dye, distinguish between different b) Differential stain: It requires more than one dye, distinguish between different types of bacteria, giving them different colours and between structures inside or outside of bacterial cell e.g. spores or capsules.

Gram stain is the most important differential stain in clinical microbiology It divides bacteria into two main categories Gram positive ( violet or purple in colour) and Gram negative (Red or pink in colour). Zeihl-Neelsen stain : It is used to identify the members of the genus mycobacterium.

Basic Structure Of Bacterial Cell All bacteria have cytoplasm containing nuclear material enclosed within a cytoplasmic membrane which in turn incoated with a cell wall .beside these basic components, some bacteria have capsules, flagellae and pili (fimbriae). The cytoplasmic membrane, cell wall are termed as cell envelope structures.

I-Cell Envelope Structures: A-Cytoplasmic membrane:- It consists of phospholipids bilayer . within which are embedded proteins of various sizes and composition. It lacks sterol (except in mycoplasma).

Function of Cytoplasmic Membrane:- Selective transport of nutrient elements either by simple diffusion in response of concentration gradient or active transport against concentration gradient & this requires energy. Respiratory activity (electron transport & oxidative phosphorylation). Bacterial multiplication: during bacterial division, it forms a septum separate the two new bacterial cells .Also, invaginated areas of the cytoplasmic membrane are called the mesosomes they are the site of chromosomal attachment during cell division..

Excretion of extracellular enzymes to digest large molecules e. g Excretion of extracellular enzymes to digest large molecules e.g. hydrolytic enzymes ,and to destroy harmful substances like antibiotics e.g. penicillin degrading enzymes. Chemotactic system: responding to surface messages by attractants and repellants e.g. motile bacteria direct their movement to glucose .

B-Cell Wall : This is the outer most covering layer of the bacterial cell. It is strong but has some elasticity . Its strength is due to (peptidoglycan) containg mainly: N- acetylglucosamine and N-acetylmuramic acid connected by polypeptide cross links. This peptideglycan layer differentiates Gram positive and Gram negative bacteria.

Function Of Cell Wall: Maintains the characteristic shape of bacteria. Supports the cytoplasmic membrane . It protects the cytoplasmic membrane & the bacterial cell contents from environmental hypotonicity.

Plays a role in bacterial multiplication. Responsible for the difference in staining reaction . Have special receptors for Bacteriophages Target for many antimicrobials. It's antigens (Lipo-polysaccharide & teichoic acid ) are used in laboratory identification.

II- External Appendages 1- Flagella: Motile bacteria have flagella too small to be detected by light microscope. They are demonstrated clearly by electron microscope. They are filamentous structures which arise in the cytoplasm, &pass through the cell wall. They are protein in nature & antigenic. There are called organs of motility.

2- Fimbria (pili). These are fine surface appendages, protein in nature usually in gram-negative bacilli ,and play an essential role in the adhesion and of no role in bacterial cell motility. There are two types of pili :

Ordinary pili: They are antigenic, also called colonization antigens which enable the bacteria to adhere to specific receptors on human cell surface. Sex pili: Long hollow special tube which is involved in the transport of DNA between two bacterial cells. This is called conjugation pili.

The Capsule: It is an organized accumulation of material (protein or carbohydrate) outside the cell wall. Function of capsule: It has a protective role for bacteria against phagocytosis within the host body. Adherence of bacteria to certain sites e.g. dextrans aiding cariogenic bacteria to establish themselves on the teeth. Important in identification & typing of bacteria e.g. in pneumococci.

III-Cytoplasm & Intracytoplasmic Structures: This is a soft gel ,containing a large amount of RNA which is collected into granules called ribosomes responsible for the manufacture of bacterial enzymes & proteins .In certain types of bacteria ,the cytoplasm also contains inclusion granules formed of lipid ,glycogen or poly-metaphosphate as the metachromatic granules present in diphtheria bacilli.

Nucleus: It is composed of DNA in the form of a single circular chromosome carrying several genetic characteristics, and there is no nuclear membrane or nucleolus.

Plasmids: These are extra chromosomal DNA molecules. They are capable of autonomous replication and carrying genetic information for several characters e.g. resistance to drugs, toxin, production, and enzyme synthesis….etc. They are transmissible to other bacterial cells.

Spores: Under unfavorable conditions, certain types of bacteria such as Clostridium & Bacillus group develop a highly resistant – phase called spore. The spore is composed of a hard outer wall containing the nucleus & part of the cytoplasm ,ribosomes ,in thick cortex and coat. When the condition becomes favorable, it germinates and gives rise to the vegetative form (active form).

Assignment Define the following:- Bacterial Capsule. Flagellae Plasmid Mesosomes