I don't mean if a bacterium can be the cause of cancer inside a human. But can actually a bacterium changes in the way as normal cells change into tumor cells? So gaining such characteristics of a tumor cell and if inside human body create a tumor mass?
You ask two questions:
- If a bacteria can get the same characteristics as tumor cells.
- If such "bacterial tumor cells" actually can form a "tumor" within the human body.
I assume that your question is not if bacteria can change in such a way that the can be confused with eukaryotic cells. This they cannot do. Bacteria cannot form organelles. Because they have no organelles they are solely dependent on their membrane for energy production. Because the surface area to volume ratio decreases with cell size there is a physical possible maximum size for bacteria. Here is some more in depth info on the subject.
So what characteristics can bacteria then have that tumor cells also have? Well, according to wikipedia, a tumor is a mass of abnormal and excessively grown tissue (neoplasm) which may or may not be malignant. According to this definition bacteria could be tumor-like if the could form tissue and if this tissue could then grow abnormally or excessive.
And guess what? If you define tissue as "an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function." like Wikipedia does. Then you could argue that the biofilms some bacteria form are quite tissue-like since biofilms also contain an extracellular matrix, the cells within have the same origin and (in some cases) the bacteria can have differentiated functions.
However, the fact that bacteria can form biofilms does not make them tumor like. For that comparison to work mutated bacteria should be able to outgrow the rest of the biofilm. And while mutation that cause noncooperative/cheating/abnormally growing bacteria in biofilm are known, this kind of mutations cause the biofilm as a whole to grow slower. (source) In this sense comparing a biofilm to a colony of ants may be more appropriate than comparing it to tissue.
So to answer your questions:
Bacteria can form tissue-like formations, but when members of this formation do not cooperate anymore this typically leads to less growth, instead of increased growth.
Biofilms do form within humans. For example the bacterium Pseudomonas aeruginosa is assosiated with cystic fibrosis. And while cheaters exist among these bacteria, biofilms with high ratios of cheaters have a lower fitness. see this paper Therefor you would expect most biofilm within humans to be more tissue-like and less tumor-like.
Depends on how you define a "cancer".
The main hallmark of a cancer is cells with an agenda that can be summarized as "hungry, hungry, split".
Aggressive bacterial infections exhibit this behavior, digesting their host from the inside and quickly multiplying. Often, they spread quickly, which could be compared to going metastatic.
However, cancers have 2 advantages because they're technically human cells. This helps them dodge immune reactions, and makes a chemical "antibiotic" type treatment also difficult because it would be poisonous to regular human cells by definition. All of the known regular (non-personalized, non-immunotherapy) chemo treatments cause painful amounts of friendly-fire damage to regular cells.
Bacterial infections don't have this advantage - if they want to dodge immune reactions they have to figure it out on their own, and bacteria are so different from human cells that it's easy to make a chemical that selectively damages them with minimal side effects for humans.
Another cancer-like behavior that's possible in bacteria - benign to malignance. For example, suppose there's a symbiotic bacteria that mutates (or more likely, picks up a malicious plasmid) that turns it into an aggressive pathogen.