Ulcers may appear, the culprit, Aeromonas/Pseudomonas. They begin as reddish spots on the skin of the fish and will eventually turn into large holes. They turn inward and infect internal organs as well. Often hidden on the underbelly of the fish we do not see them until the ulcer is in a more developed and dangerous stage. Septicemia can occur. This infection is internal and can cause your fish to spiral instead of swim. 

Bacterial Dropsy is another example of bacteria attacking from within. The eyes of the fish bulge as does his body and the scales along his side may stick out like a pinecone. More than likely the bacteria of one of the Big Four has entered the Koi's system.

Bacterial Gill Disease causes the fish to gasp for air and hang just beneath the surface or under a waterfall. If the inside of the gill is a brown/green color it is more than likely this disease. Another cause would be Gill Flukes. They have the same symptoms but with out the infection part. Irregardless, Gill Flukes can also cause the Bacterial Gill Disease. Flukes in general can attach to any part of the fish causing an ulcer which only leads to further problems.
     In extreme cases you can smell bacteria. It smells bad. If your pond smells like sulfur or rotten eggs it’s a good bet that bacteria is running rampant in your system. You need to rectify this immediately.

How To Prevent Bacterial Outbreaks:
There’s nothing better to prevent bacterial complications than a clean, healthy pond environment.
Here are some tips to help protect your fish from the threat of bacterial infections:

Do Not Over-Crowd the Pond
When you crowd too many people into a room someone will get sick, even if no one was ill before coming in. Everyone needs a little elbow room, even fish. Stress, like in humans  is a major factor in inducing disease in fish. Try to avoid stressful situations and conditions. Stress by itself through overcrowding or simply moving fish from one pond to another can sometimes be enough to allow a bacterial infection to start.

Keep Excellent Water Quality
Imagine living, eating and sleeping in a smoke filled room. (Some of us do!) We have the option, however, of going outside for a breath of fresh air. Fish do not. Their water is their living, eating and sleeping environment. Keep it clean, ammonia and nitrite free! Keep up good water circulation. Water within the pond environment that is stagnant or slow moving is an ideal environment for bacteria to flourish. Add fresh water. To dilute the ratio of bacteria to water, replace approx. 10% of water on a weekly basis and they will grow faster too. Don't forget a good filtration system that will help keep the pond balanced. Poor water quality can burn off the slime coat or stress the koi making it more susceptible to the bacteria. Click Here for more on Water Quality and the Nitrogen Cycle.