Bacterial Basics: Gram Staining, Morphology, and Reproduction

Gram Staining: Positive and Negative Bacteria

In bacteria, the cell wall is a determinant of cell shape and has also served as a classification criterion. In 1884, French bacteriologist Christian Gram developed a method to observe bacteria under light microscopy using specific staining. However, not all bacteria are stained with this method, which led to their classification into two groups: Gram-positive bacteria, which retain the stain, and Gram-negative bacteria, which do not retain the stain.

This distinction reflects structural differences in the bacterial cell wall. In Gram-positive bacteria, the wall is mainly composed of peptidoglycan and teichoic acid. In contrast, Gram-negative bacteria have, in addition to peptidoglycan, an outer membrane containing molecules of lipoproteins and lipopolysaccharides.

Bacterial Arrangements and Grouping

Many bacterial species, especially cocci, remain together after division, forming characteristic arrangements that facilitate identification.

• When cocci divide in one plane and remain in pairs, they form diplococci (e.g., Neisseria, pneumococci). Neisseria are often described as kidney-shaped or "coffee bean" shaped diplococci, joined at their flattened sides. Pneumococci are typically elongated or lance-shaped diplococci.
• If cocci divide in one plane and remain attached in a linear fashion, they form a chain or streptococcus.
• If cocci divide in multiple planes, they form irregular clusters resembling bunches of grapes, characteristic of staphylococci.

Bacilli are less likely to remain together after division, but some arrangements are noted:

• Chains of bacilli (e.g., Bacillus species, sometimes referred to as streptobacilli).
• Bacteria arranged in a "palisade" or "picket fence" formation, where rods are joined side-by-side or end-to-end after division.

Characteristics of Bacteria

Bacteria are among the smallest and structurally simplest living organisms. They are prokaryotic cellular organisms, meaning they lack membrane-bound organelles and a nucleus. The absence of membrane-separated compartments results in metabolites being disseminated throughout the cytoplasm.

Despite their structural simplicity, bacteria are biochemically complex and diverse, enabling them to adapt to a wide variety of conditions. Most bacterial cells are very small. While most prokaryotic bacteria are unicellular, some form colonies or filaments, with some degree of cell specialization.

Although many species have irregular shapes, bacteria generally exhibit a few basic forms:

• Cocci (spherical)
• Bacilli (rod-shaped or cylindrical)
• Spirochetes (spiral)
• Vibrios (comma-shaped)

Cocci may exist as single cells or form groups like diplococci (pairs) or staphylococci (irregular clusters).

Bacteria exhibit various mechanisms of nutrition: Some are heterotrophic, feeding by absorbing nutrients, while others are autotrophic, performing photosynthesis or chemosynthesis.

All bacteria reproduce asexually.

Bacterial Reproduction

Bacteria, like other single-celled organisms, primarily reproduce by simple division, also known as binary fission. During this process, the cell replicates its DNA and then divides into two genetically identical daughter cells, distributing cellular components relatively equally.

Starting from a single parent bacterium, two daughter cells are generated. If each of these divides, there will be four, and so on. Under optimal environmental conditions (nutrients, temperature, etc.), the number of bacteria can increase exponentially over time (e.g., 1, 2, 4, 8, 16...).

Most bacteria can divide rapidly, often in less than an hour under optimal conditions. However, some species, such as those causing leprosy or tuberculosis, have much slower reproduction rates.