Planet Earth is approximately 4.5 billion years old. The evidence for microbial life can be found in rocks 3.86 billion years old. Microbes were, so to speak, the first colonizers of the earth.
Bacterial cells are 10 times smaller than human cells. They have all different kinds of shapes: they can be round, long, curly, have tails, hair and sex-pilii, a tube formed by bacteria to connect to each other in order to exchange genetic information.
Bacteria can have sex?!
Yes, bacteria can have sex; they connect to each other and exchange genetic information (just to mention, this is one way how bacteria spread antibiotic resistance). They can swim, attach to surfaces and build huge cities (biofilm). Bacteria also talk to each other via certain chemical languages, telling their conspecifics how much nutrients are left or if any enemy is lurking nearby.
They teach our immune system to stay calm when only the good bacteria are present, and they fight side-by-side with our immune system against the pathogenic bacteria by releasing antibacterial peptides and by occupying our body space so that pathogenic bacteria can't colonize.
Bacteria produce vitamins for us
They are our personal power plants: 30% of the calories we process from food are processed by the bacteria in our gut. What’s more, bacteria produce vitamins for us, for example vitamin K.
How are they able to do all these things?
Well, the secret is the great diversity of the bacteria. They all come together, each species with its specific tool (genes); in sum, 150 x more genes than the human body.
In contrast to our eukaryotic cells (human cells, animal cells, etc.), bacterial cells don't have cell-organelles or a nucleus. Everything in the bacterial cell is located in only one compartment, the cytoplasm.
This is how bacteria can look like in detail:
Dr. Hans Christian Gram, developed the Gram stain
The bacterial cell wall is a very important classification feature of bacteria. In 1884, the Danish medical doctor Hans Christian Gram, whilst working at a hospital in Berlin, wanted to make certain bacteria stand out from others. As bacterial infections were killing thousands of people every year, he had hoped to identify the bacteria causing the infections. He therefore decided to mark them with a colorant. To his great surprise, many strains could not be colored, whereas others could. Dr. Gram delved deeper into the matter and discovered that there are bacteria with a rigid, thick cell wall and a higher inner cell turgor – gram-positive bacteria. Gram-negative bacteria have a thinner cell wall but an additional protective outer membrane. The latter group could not be marked by the colorant, thus we now call them Gram-negative because they do not respond to the method of marking invented by Dr. Christian Gram. His method today is the most important classification tool in microbiology. It just so happens that precisely these Gram-negatives are the most resilient to antibiotics – they harbour a natural antibiotic resistance. You may have even heard some of the names from these groups before: E. coli, Pseudomonas, Acinetobacter and Salmonella, for example. The Gram-positives also have quite prominent members, such as MRSA. Nevertheless, the bad bacteria are the most famous, but in numbers a tiny fraction to the vast variety of bacteria.
The enormous genetic diversity of bacteria is mirrored by an equally enormous metabolic diversity. Each bacterial species is specialized for the exploitation of nutrients from their environment. While one species is happy living in a distinct economic system, the other species cannot survive because they can't utilise the available nutrients at this site.
Bacteria can undergo photosynthesis, fermentation, aerobic and anaerobic respiration, denitrification, methanogenesis…; they use water, oxygen, carbon dioxide, hydrogen sulphide, nitrogen, iron, sugars …
This wide range of metabolic pathways is possible through diverse enzymes which are encoded by diverse genes. These enzymes are not only tools for the bacteria, they are also essential for their environment and for us!
While the bacteria break down the nutrients necessary for their own survival, the by-products they release are used by the host: you and me!