Origin of Bacteria, Classifications and Diseases Caused

 

On Thursday, September 19, 2024, I wrote an essay on:

Origin of Viruses, Viral Diseases and Monkey Pox

As promised, I shall now continue with bacteria- induced diseases. Let me fleetingly start with their origin.

Origin of Bacteria

Bacteria are among the oldest life forms on Earth, dating back over 3.5 billion years. The exact origin is still a topic of debate. There are at least three prevailing hypotheses.

The first one is the Primordial Soup Hypothesis. It is hypothesized that bacteria may have originated in Earth's early oceans, where chemical reactions between simple molecules gave rise to life. Over time, these early life forms evolved into more complex bacteria.

The second one is the Panspermia Hypothesis where it was suggested that bacteria (or their precursors) might have originated from space, arriving on Earth via meteorites or comets.

The third proposal is the Hydrothermal Vent Hypothesis. Evolutionary biologists have hypothesized that bacteria might have evolved near deep-sea hydrothermal vents, where extreme conditions allowed for unique forms of early life to develop.

Having hypothesized the origin of bacteria, let us go into their classification.

Bacteria are diverse and can be classified based on several characteristics:

First, are their shapes. Here are their shapes and appearances.

1. Cocci are spherical (e.g., Staphylococcus)

2. Bacilli are rod-shaped (e.g., Escherichia coli)

3. Spirilla are spiral-shaped (e.g., Spirillum minus)

4. Vibrios are comma-shaped (e.g., Vibrio cholerae)

Secondly, are their colours after Gram staining. By this I mean a technique we use to differentiate bacteria based on the structure of their cell walls. Gram-positive bacteria show thick peptidoglycan cell wall, retain purple colour (e.g., Staphylococcus aureus), whereas gram-negative bacteria show thin peptidoglycan layer, appear pink after staining (e.g., Escherichia coli).

Thirdly, are their oxygen requirements. Aerobic bacteria require oxygen for growth (e.g., Mycobacterium tuberculosis), whereas anaerobic bacteria grow in the absence of oxygen (e.g., Clostridium botulinum). Facultative anaerobes can grow with or without oxygen (e.g., E. coli).

What about their metabolic characteristics? Phototrophs obtain energy from light (e.g., Cyanobacteria), whereas chemotrophs derive energy from chemical compounds (e.g., Nitrosomonas).

Cell structure of prokaryotic are those bacteria that lack a membrane-bound nucleus and other organelles. This differentiates them from eukaryotes (e.g., human cells).

Classification by Phyla:

1.      Proteobacteria are a large group including E. coli and Salmonella. 2.

2.      Firmicutes include Clostridium and Bacillus.

3.      Actinobacteria include Mycobacterium.

4.      Cyanobacteria are photosynthetic bacteria responsible for producing oxygen in Earth's early atmosphere.

Having briefly explained the above, let us now have a look at the diseases they cause (Pathogenicity of Bacteria). This area would be of interest to medical doctors who specialize in infectious diseases.

Some bacteria are pathogenic, meaning they cause disease, while others are commensal or mutualistic, meaning they coexist harmlessly with their hosts or even provide benefits (such as gut bacteria). Bacteria can become pathogenic through several mechanisms. We shall briefly list at least four (4) below.

1. Toxin Production - Some bacteria produce toxins that damage host tissues or disrupt normal cellular processes. Examples include exotoxins released by bacteria (e.g., Clostridium tetani produces tetanus toxin). Endotoxins are found in the outer membrane of Gram-negative bacteria (e.g., E. coli endotoxin).
2. Invasion of Host Cells -   Pathogenic bacteria can invade and replicate within host cells, leading to tissue damage and immune responses.
3. Immune Evasion - Some bacteria have evolved mechanisms to avoid detection or destruction by the host’s immune system. For example, Mycobacterium tuberculosis survives within immune cells.
4. Opportunistic Infections - Certain bacteria are usually harmless but can cause disease in immunocompromised individuals (e.g., Pseudomonas aeruginosa).

Examples of pathogenic bacteria and diseases they cause.

1. Escherichia coli (E. coli).  They are Gram-negative, facultative anaerobe. The diseases they cause are gastrointestinal infections, urinary tract infections (UTIs). The symptoms presented are diarrhoea, abdominal cramps, fever.  Most E. coli infections are self-limiting; severe cases may require antibiotics.
2. Mycobacterium tuberculosis (TB). These are Gram-positive, aerobic, and the disease they cause is tuberculosis where the symptoms presented are persistent cough, weight loss, night sweats, fatigue. Tuberculosis is treatable with a long course of antibiotics (e.g., isoniazid, rifampicin), but antibiotic-resistant strains pose a challenge.
3. Staphylococcus aureus (MRSA). These are classified as Gram-positive, facultative anaerobe. The diseases caused are skin infections, sepsis, pneumonia. Symptoms presented are red, swollen boils or abscesses, fever. It is curable with appropriate antibiotics, though MRSA strains are resistant to many drugs.
4. Clostridium botulinum (Botulism). The classification is Gram-positive, anaerobic, and the disease they cause is botulism. The symptoms are paralysis, difficulty breathing, muscle weakness. This can be treated with antitoxins and supportive care; curable if treated early.
5. Vibrio cholerae (Cholera). The classification is Gram-negative, facultative anaerobe. The disease is cholera with symptoms of severe diarrhoea, and dehydration. Cholera is curable with rehydration therapy and antibiotics (e.g., doxycycline).
6. Helicobacter pylori (Stomach Ulcers). Here we classify it as caused by Gram-negative, microaerophilic. The disease caused is gastric ulcers and gastric cancer. The symptoms are stomach pain, bloating, nausea. Ulceration of the stomach is curable with antibiotics and proton pump inhibitors (PPI).

Let us now have a look at bacteria that do not cause disease (non-pathogenic bacteria).  Many bacteria are beneficial and play critical roles in human health and the environment. Examples include gut microbiota.  Bacteria in the human intestines (e.g., Bacteroides, Lactobacillus) aid in digestion, vitamin synthesis, and immune regulation.

Nitrogen-fixing bacteria are Rhizobium species that convert atmospheric nitrogen into forms usable by plants, crucial for agriculture.

The next question someone has asked me is, why are some bacteria pathogenic? There are 3 factors that I know.  

1. Virulence factors. This means pathogenic bacteria possess specific genes or mechanisms (e.g., toxins, adhesins, enzymes) that enable them to cause disease.
2. Host susceptibility.  Weakened immune systems, poor hygiene, and other factors increase the likelihood of bacterial infections.
3. Environmental factors. Bacteria thrive under certain environmental conditions, like temperature, pH, or the presence of nutrients, which can enhance their pathogenic potential.

Bacteria vs. Viruses

First, is the size.  Bacteria are much larger than viruses and can be seen with a light microscope. Viruses are typically visible only under an electron microscope.

Second, is the cell structure. Bacteria are living, single-celled organisms with a cell wall, plasma membrane, and the ability to replicate independently. Viruses are non-living, lack cellular structure, and require a host to replicate.

Third, are antibiotics that are effective against bacteria, but not viruses. Antibiotic misuse can lead to bacterial resistance.

Others have asked me about the curability of bacterial infections.

Most bacterial infections are curable with antibiotics. However, the rise of antibiotic-resistant bacteria (e.g., MRSA, multidrug-resistant TB) presents a significant challenge in modern medicine. The development of new antibiotics and alternative treatments (e.g., bacteriophages, immunotherapy) is essential for managing resistant strains.

Bacteria are incredibly diverse, with some species being essential to life and others capable of causing serious diseases. Understanding their origin, classification, and pathogenic mechanisms helps us develop effective treatments and preventive measures.

I shall in the next two articles over the week write on parasitic diseases, and diseases caused by moulds and micro-fungi.

Hope this is educational. Take Care with communicable diseases.