This method is designed to help new players win their first game quickly: Bacteria on Normal with no genes.

Start in Saudi Arabia.
Buy the following in this order as soon as you have enough DNA to afford them (NOTE: devolve any lethal symptoms that randomly mutate until every country is infected):
Symptoms
1. Rash
2. Sweating
3. Skin Lesions
Transmissions
4. Air 1
5. Water 1
6. Air 2
7. Water 2
8. Extreme Bio-Aerosol
Abilities
9. Cold Resistance 1
10. Cold Resistance 2
11. Drug Resistance 1
12. Drug Resistance 2
13. Heat Resistance 1

Once every country is infected, it's time to ramp up infectivity and lethality. Buy these:
Symptoms
14. Necrosis
15. Fever
16. Immune Suppression
17. Total Organ Failure
18. Hemorrhagic Shock
19. Pulmonary Fibrosis
20. Pulmonary Oedema
21. Diarrhoea
22. Dysentery
23. Internal Hemorrhaging
24. Tumours

All you have to do now is wait until everyone dies.

Here is one method to win as Bacteria that usually works even on Mega Brutal with no genes active:

Start in Saudi Arabia
Buy the following in this order as soon as you have enough DNA to afford them (NOTE: devolve any symptoms that randomly mutate until you have 19. Necrosis):
1. Rash
2. Sweating
3. Skin Lesions
4. Coughing
5. Pneumonia
6. Insomnia
7. Paranoia
8. Air 1
9. Water 1
10. Air 2
11. Water 2
12. Extreme Bio-Aerosol
13. Cold Resistance 1
14. Cold Resistance 2
15. Drug Resistance 1
16. Drug Resistance 2
17. Heat Resistance 1
18. Seizures
19. Necrosis
20. Pulmonary Fibrosis
21. Total Organ Faillure
22. Insanity
23. Coma
24. Paralysis
25. Dysentery

NOTE: It is still possible to lose due to random events, like if a country starts executing or bombing the infected. They are very annoying and I have not found a good way to deal with them. There's also a small chance that Greenland might not get infected.

The game doesn't explain everything in the tip popups so here is a brief explanation of the logic behind the method outlined in section II.. You may be wondering: "Why should I take symptoms at the start of the game? Isn't it safer to stay undetected by not evolving symptoms until all countries are infected?" These are both good questions. You should take symptoms at the start of the game because you need some Severity to win on Mega Brutal. DNA costs on Mega Brutal increase as the percentage of infected world population increases. It is not safer to try to stay undetected on Mega Brutal because you will eventually be detected even if you never evolve any symptoms AND you will not have enough DNA to evolve enough lethal symptoms to win the game. Severity is the most important attribute when it comes to winning as Bacteria on Mega Brutal. As Severity increases, you get more DNA from popping bubbles BUT countries also discover you faster and they close airports/borders/seaports faster. I have tested many Bacteria games on Mega Brutal with no genes and the most effective and reliable method I have discovered was the one outlined in section II..

The main purporse of this guide is to allow even the newest players to beat Bacteria on Mega Brutal. However, I will add more sections to this guide if people are interested in maximizing their potential with genes for specific difficulty levels and/or speed runs. I know from personal experience that it's possible to beat Bacteria on Mega Brutal in less than a year.

I hope you've enjoyed my guide and that I've explained everything adequately. Please feel free to ask me anything about this guide or about Bacteria. :)

-BMcC