Bacteria are prokaryotes—the smallest, simplest and most ancient cells, with free-floating genetic material. These microscopic single-celled organisms can be rod, spiral or spherical in shape.
There are two types of bacteria: Gram-negative and Gram-positive. The key difference is the presence of an extra outer membrane in Gram-negative bacteria. It’s essentially an extra line of defence that makes it harder for antibiotics to penetrate, thus making Gram-negative bacteria more difficult to kill and more prone to developing resistance.
Bacteria are abundant in soil, inhabiting plant root systems to provide services like nitrogen fixing or acting as antifungal agents. Thermophilic (heat-loving) bacteria fix sulphur to produce sulphide and energy for photosynthesis in aquatic sediments or organically rich waters.
Dangerous bacteria also live in soil, a good reason to wear gardening gloves. The floods in northern Queensland in 2019 brought Burkholderia pseudomallei to the surface, bacteria which cause a serious infection known as melioidosis.
In our bodies, bacteria inhabit the human digestion system, live on our skin and contribute to energy metabolism, digestion, brain function and general wellbeing. But if the balance of these bacteria is tipped by a dose of antibiotics or ill-health, then gut discomfort or skin infections are common.
Infectious diseases caused by bacteria have killed well over half of all humans who have ever lived on Earth. Historically, bacterial infections have started major pandemics such as the bubonic plague, which is estimated to have killed 50-60 per cent of the population of Europe during the Black Death in the 14th Century.
Bacteria reproduce mainly by binary fission–replicating their DNA so they have two copies on opposite sides of the cell, then growing a new cell wall down the middle to produce two daughter cells. This doubling time takes between 20 minutes and an hour.
This short generation time allows mutations to emerge and accumulate rapidly and quickly cause significant changes in bacteria, such as resistance to antibiotics.
Communication is key
Bacteria can communicate with one another by releasing chemical signalling molecules, allowing the population to act as one multicellular organism.
Depending on the density of molecules and the signal it generates, the bacterial community can adapt and respond to compete for resources in a process known as quorum sensing.
This ability to communicate with one another allows bacteria to coordinate gene expression, and therefore the behaviour, of the entire community.
This process gives bacteria some of the qualities of higher organisms and is a powerful weapon against antibiotics. It can trigger some bacteria to shut down and become dormant when exposed to an antibiotic, and they are able to regenerate when the antibiotic is gone.
What are viruses?
Viruses are an assembly of different types of molecules that consist of genetic material (either a single- or double-stranded DNA or RNA) with a protein coat and sometimes a layer of fat too (an envelope).
They can assume different shapes and sizes—spacecraft designs, spirals, cylinders and ball shapes.
Viruses that are enveloped with a layer of fat (such as SARS-CoV-2 which causes COVID-19) can be more readily killed by simple handwashing, because soap disrupts this fatty layer.
Viruses can’t reproduce on their own (unlike bacteria) so they aren’t considered ‘living’, but they can survive on surfaces for a varying level of time.
Viruses cause diseases including the flu, herpes simplex virus, Ebola, Zika and the formidable common cold.
Viruses can be quite selective about where they live and reproduce–many viruses don’t even infect humans. Some viruses only infect bacteria, some only infect plants, and many only infect animals.
However, a virus can evolve to jump into humans. This often happens with influenza: for example bird flu or swine flu which originated in birds and pigs and managed to infect humans. SARS-CoV-2, the virus that causes COVID-19, probably jumped into humans from bats.
The life cycle of a virus can be divided into the following stages: entry of the virus into the host cell; replication of the viral genome; production of new viral proteins; assembly of those viral proteins into new viruses and then release from the host cell (either by killing the cell or by budding off the host cell membrane) ready to infect new cells.