Bacterial Structure

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Last modified 18/02/2015

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Chapter 6

Bacterial Structure

Bacterial Morphology

Overview

1. Bacteria are prokaryotes

2. The differences between eukaryotic and prokaryotic cells, summarized in Table 6-1, are the basis for antimicrobial drugs.

TABLE 6-1

Prokaryotic Versus Eukaryotic Cells

Characteristic Prokaryotic Cells Eukaryotic Cells
Size (approximate) (μm) 0.5–3 >5
Cell wall Complex structure composed of proteins, peptidoglycan, and lipids Only in fungal and plant cells; composition differs from that of bacterial cell wall
Plasma (cytoplasmic) membrane Contains no sterols (except in Mycoplasma species) Contains sterols
Nuclear membrane Absent Present
Genome Single, circular DNA molecule in nucleoid Multiple, linear DNA molecules in nucleus
Organelles* Absent Present
Ribosomes 70S (50S + 30S subunits) 80S (60S + 40S subunits)
Cell division Via binary fission Via mitosis and meiosis

*Include mitochondria, Golgi complex, and endoplasmic reticulum.

Size of bacterial cells

Shape and arrangement of common bacteria (Fig. 6-1)

Gram staining

II Bacterial Ultrastructure

• Gram-positive and gram-negative bacteria have similar internal structures but structurally dissimilar cell envelopes (Fig. 6-2; Table 6-2).

Internal bacterial structures

1. Nucleoid is the central region of bacterium that contains DNA.

2. Bacterial cells contain a single chromosome composed of a circular DNA molecule.

3. Because bacteria lack a nuclear membrane, transcription and translation are coupled (i.e., ribosome-mediated protein synthesis can begin while a messenger RNA [mRNA] is being produced and is still attached to the DNA).

4. Bacterial ribosomes differ in size, components, and shape from eukaryotic ribosomes and thus are a major target of antibiotic action.

5. Plasmids, which are small, circular fragments of extrachromosomal DNA, may be present and often carry antibiotic resistance genes.

Cell envelope (Table 6-3)