What Is a Bacterial Colony?

Invisible to Visible

A single bacterium is far too small to see with the naked eye. But give that bacterium some nutrients, warmth, and time, and it will divide again and again until millions of its descendants pile up into something you can actually see — a colony.

Think of a bacterial colony as a tiny city, built by one founding citizen that kept duplicating itself. All the bacteria in a colony are clones of the original cell — genetically identical copies sharing the same ancestry.

From One to Millions

Here's the math: If a bacterium divides every 20 minutes, after just 24 hours that single cell would theoretically produce over 4 sextillion (4 × 10²¹) cells! In practice, nutrients run out and waste products accumulate, but a typical visible colony still contains between 1 million to 1 billion cells.

Why Colonies Matter

Colonies are essential for microbiology because they allow scientists to:

• Isolate pure cultures: Each colony (usually) comes from a single cell, so all bacteria in it are the same species. This is crucial for studying specific bacteria.
• Identify bacteria: Different species form colonies with distinctive appearances — size, shape, color, texture, and smell can all provide clues.
• Count bacteria: By spreading a diluted sample on a plate and counting colonies, scientists can estimate how many bacteria were in the original sample.
• Test antibiotics: Growing bacteria on plates allows researchers to test which drugs can kill them.

Colony Characteristics

Microbiologists describe colonies using several characteristics:

• Form (shape): Circular, irregular, filamentous, or rhizoid (root-like)
• Elevation: Flat, raised, convex, or umbonate (with a raised center)
• Margin (edge): Entire (smooth), wavy, lobate, or filamentous
• Size: From pinpoint to several centimeters
• Color: Many bacteria produce pigments — golden Staphylococcus aureus, red Serratia marcescens, or green Pseudomonas aeruginosa
• Texture: Smooth (shiny), rough, mucoid (slimy), or dry
• Opacity: Transparent, translucent, or opaque

The Petri Dish: A Microbial Garden

Colonies grow on solid growth media in petri dishes (named after German bacteriologist Julius Richard Petri). The medium is usually agar — a gel-like substance derived from seaweed — mixed with nutrients. Different media can select for different bacteria or make certain species stand out by color.

Beyond Bacteria

It's not just bacteria that form colonies:

• Fungi form fuzzy, often colorful colonies (think of mold on bread)
• Yeasts form smooth, creamy colonies similar to bacteria
• Archaea can also form colonies, though they're less commonly cultured

A Colony Is More Than a Clump

Recent research shows that colonies aren't just random piles of cells. Bacteria in colonies can communicate through chemical signals, share nutrients, and even specialize — with cells on the edge behaving differently from those in the center. Some colonies form intricate patterns and structures. In this sense, a colony is more like a primitive organism than just a collection of individuals!