Bacteria

Bacteria are fascinating, microscopic organisms. One of the most fascinating aspects about them is their duality; they are both extraordinarily good for us and simultaneously terribly bad for us. This duality is dependent on the specific bacteria, the time, the place, and the function in that place. We need to recognize the features of specific bacteria; from their gram stain, to their primary method of metabolism, to what they contribute to our ecosystem. This helps us to categorize bacteria individually and to have a better understanding of bacteria as a group and microbiology as a subject.

What are Bacteria?

Bacteria are defined as a group of unicellular microorganisms, some of which cause disease, that is distinguished by the following features - the presence of a cell wall, the lack of discrete organelles, and the lack of an organized nucleus.

Bacteria lack organelles and nuclei

In the great division of living organisms between prokaryotes and eukaryotes, bacteria are firmly in the prokaryotic category.

The reason bacteria do not have organelles or nuclei is not that they lack genomes, or lack enzymes and enzymatic activity that might otherwise be found within an organelle in eukaryotes (such as humans). Bacteria have genomes, certainly, and have enzymes ranging from urease to penicillin-binding proteins that can lead to penicillin resistance in certain infections. However, bacteria lack organelles and any nucleus because they lack internal membranes within their cells. The sole membrane typically present in bacteria is their cell/plasma membrane. This is in contrast to humans and other eukaryotes that have membranes for their lysosomes, their mitochondria, their peroxisomes, and many other organelles found within them.

Bacteria structure

We know the differences between the structure of a gram-negative and a gram-positive bacterial cell wall. But what about the other components that make up the ultrastructure of the bacterium?

We will discuss these structures from the outermost to the innermost, so we can have a good idea of each component's placement and location, as well as its function. Remember that not all these features are common to all bacteria.

• Flagella - these are attached to and located on the outside of the capsule (we'll get to that in a second!). The primary function of flagella is movement, they help facilitate chemotaxis, which is when an organism moves closer to some chemical stimulus (often food). Flagella cause a propeller-like movement that helps the bacteria to tumble and swim.

  • Flagella are made of flagellin - a protein.

    • These flagellin subunits are joined together to form a hair-like tube that's hollow on the inside.

  • Attaching the flagella o the capsule is a motor,

  • Flagella can be monotrichous - a single flagellum, located at one side, the end of a bacterium.

  • Flagella can be peritrichous - multiple flagella, located all over the bacteria.

  • Flagella can be lophotrichous - multiple flagella, located at one side, the end of the bacterium.

• Pilus - also known as the sex pilus, these may be mistaken for flagella at times but they have very different uses and structures. Pili help bacteria to do conjugation, also known as bacterial sex, where virulence genes are passed from one bacteria to the next. These genes are passed typically, on a plasmid (we'll get to that in a bit!).

• Capsule - this is a layer on the outside of bacteria, made of polysaccharides.

  • Capsules are often considered virulence factors, which are traits that increase the bacteria's ability to infect and its pathogenicity. This is because capsules help bacteria evade phagocytosis.

• Cell wall - we will elaborate on the bacterial cell wall below. It is made of peptidoglycan and differs in thickness based on whether the bacteria are gram-negative or gram-positive.

• Plasma membrane - this is a lipid bilayer membrane. Like our own plasma membranes, they are semi-permeable and allow for certain solutes to pass through.

  • Mesosome - although this is may be listed as a special component of bacteria, it is just an invagination of the plasma membrane. This is not a native structure of bacteria, but rather these invaginated folds are created during certain chemical fixing processes that scientists use to visualize bacteria.

• Cytoplasm - the cytoplasm of bacterial cells is actually rather thick, and gel-like. It's the central location for many of the enzymes, ribosomes, proteins, nutrients, and assorted cell structures of the bacteria.

• Plasmid - this is extra, circular, double-stranded DNA that a bacteria may have within its cytoplasm. Typically, the genes on a plasmid promote virulence and antibiotic resistance, and they can be passed from bacteria to bacteria through the pili.

  • Plasmids replicate independently of the bacterial genome properly.

Fig. 1. Bacterial cell diagram showing the basic organelles of a bacterium.

Bacteria cell: the bacterial cell wall

Bacteria have cell walls located outside of their cell membrane. The cell wall is made up of sugar (glycan) chains connected by short peptides, hence it is given the name peptidoglycan. The peptidoglycan cell walls form a dense, mesh-like structure; known as a crystal lattice, as the glycan chains connect to each other via their peptides, by cross-linking them.

There are two sugars that comprise peptidoglycan; NAG and NAM (for N-acetylglucosamine and N-acetylmuramic acid, respectively). NAM is the glycan that contains the peptide attachment; called an oligopeptide.

NAG and NAM attach to each other directly on the same chain, and then two NAM molecules on adjacent chains form a cross-link between their oligopeptides.

The cell wall functions to give the bacteria its shape, as well as to protect it from external environmental factors and lysis, two very essential functions.

Types of bacteria

Based on the features of their cell wall apparatus (or lack thereof) bacteria can be grouped into the following categories:

• Gram-positive bacteria

• Gram-negative bacteria

• Bacteria without a cell wall

Gram-positive bacteria

Gram-positive bacteria have thick peptidoglycan cell walls. This is their most distinguishing feature (as a group) and is also what allows them to stain violet on Gram Stain.

The cell wall of a gram-positive bacterium can be anywhere from 15 to 80 nanometers thick. Each nanometer is 1^-7 of a centimeter, and while this might not sound like much, in the land of bacteria and prokaryotes, this makes for quite a thick layer.

Sticking out of the cell wall, in gram-positive bacteria, are teichoic and lipoteichoic acids. The functions of these are not yet fully understood, but scientists think adherence may be one of their main purposes. They may help bacteria stick to an assortment of surfaces.

Gram-negative bacteria

Gram-negative bacteria, on the other hand, have thin peptidoglycan cell walls. Outside of these walls, they often have an outer membrane; another feature that helps distinguish them from Gram-positive bacteria. Gram-negative bacteria stain pink on gram stain.

Deeper within the gram-negative bacterial cell, there is a periplasm or a periplasmic space. This is a space filled with a rather thick, gel-like matrix, that is the site of important enzymes and proteins that promote bacterial existence.

Bacteria without a cell wall

We have waxed poetic on the importance of the bacterial cell wall, and while this is true, it is not so for every organism. Some bacteria, most notably a species of bacteria called Mycoplasma, but also many others, totally lack a cell wall. These bacteria are sometimes called gram-indeterminate, but may also be merely acknowledged as "not able to be gram stained" because they do not pick up any color - neither purple nor pink.

Often times these cell wall-less bacteria have to live in an osmotically stable environment (i.e. in humans!), and they may have different components of their cell membranes than other bacteria do.

Anaerobic bacteria vs aerobic bacteria

Bacteria can also be categorized by their method of cellular respiration (Table 1). Bacteria can be aerobic, or anaerobic. When we say respiration in this case, we are not talking about breathing with lungs; rather we are referring more to metabolism and growth. What are the substances that bacteria can take from the environment and utilize for energy, and what kind of environment can particular bacteria live in?

Table 1: Aerobic vs anaerobic bacteria.

Examples of Bacteria

A few very known examples of bacteria are Escherichia coli, Yersinia pestis, or the genus Lactobacillus. Let's look at them in more detail.

• Escherichia coli, also known as E. coli, is a type of bacteria that is frequently used in research laboratories. Most strains of Escherichia coli are innocuous to humans, in fact, they live in our intestines. However, some can cause cramps, diarrhoea, urinary infections and other symptoms.
• Yersinia pestis is at the centre of one of the most famous pandemics ever: the plague. Also known as the Black Death, the plague can be passed on through the bite of fleas that were living off an infected rodent.
• Lactobacillus is a genus of bacteria that live inside humans, for example in our gastrointestinal tract, or in the vagina. Although each strain works differently, overall they help us process food and absorb nutrients. They are also used frequently as probiotics, and they can be found in milk and milk-derived products. The most common Lactobacillus is Lactobacillus acidophilus.