Regarding the TYPE of wound infections that a soldier before the late 1800s would have, they would be the same as the types today. The microbes responsible for wound infections today are microbes that already live on our skin / soft tissues. Since it's unlikely that the human skin bacterial flora has changed considerably in the last 200,000 years, it follows that the same microbes that cause gangrene in untreated wounds today caused it in 1500s or 1000 B.C.

Here's a table showing the most common microbes:

[ur]http://emedicine.medscape.com/article/188988-overview#a7[/url]

One caveat is that the incidence of tetanus would be much higher than today considering that widespread tetanus vaccination is a product of the modern industrial world, starting in the 1890s.

https://www.cdc.gov/vaccines/pubs/pinkbook/downloads/tetanus.pdf

As far as lethality, tetanus would be by far the most lethal considering the mortality rate without vaccination or treatment is near 100% today.

As for the other bacteria, it's impossible to know since how lethal an infection is would depend on a variety of factors unique to the victim such as their immune system's characteristics, their nutritional status at the time of the infection, how extensive the wound is and how capable are the military field docs/surgeons/barbers at keeping the wound clean.