I infect mice with bacteria FOR GREAT JUSTICE for science. To do this, I first take mice and anesthetize them. Once they're out, I shave their necks and make a small incision right over the trachea. I then take a small bore needle and give a dose of bacteria directly into the trachea before sealing the critter back up and letting it recover.

I have 8 different bugs that I'm trying out at the moment. I only did two of them today, along with the vehicle control. Mice are surprisingly hard to infect with bacteria. You'd think that ~10,000,000 bacteria shot down into the lungs would be a serious problem for a little 30g rodent, but they're tough little critters. It's a real challenge to make that infection stay around more than 3-4 days, and by that time it's really just a mop-up operation. Lungs are exceptionally good at getting bad shit out of them. Fortunately (for the bugs, not the mice) people like me are there to give them (again bugs, not mice) a helping hand. I haven't quite figured it out yet, hence the small cohorts, but I'm getting there. My last vehicle actually got a 2 log increase with anaerobes. Sweet.

But I digress. I'm here to discuss the sadism of mice, not men.

I normally do them in challenge groups, completing each bacterial or control group before moving on to the next. That way I can prevent cross contamination by just swapping out my instrument sets between groups. My next-to-last group today had an interesting little twist

I had two cages left. Five in one, two in the other. I was getting ready to do the last mouse of the second to last group. I grabbed one of the mice in the cage holding only two. Normally, I'd just anesthetize both of them, but that last mouse was in the next group. If I hit him now, he'd likely be recovering by the time I got all my round 4 instruments ready.

I like to separate the anesthetized mice from the unanesthetized mice; it just makes it easier. The cages are set up nicely for this. There is a plastic bottom and a wire lid. Over this lid is the microisolator lid. It's just a big filter top on top of the mouse cage. It keeps the outside out and the inside in. It's similar in size and shape to the bottom, and when inverted makes an excellent holding area for mice that are slowly drifting off to la-la land.

I gave him his injection and set him back in the lid to go to sleep. I normally work in parallel, as one mouse is going out, I'm working on the mouse before him. So, after I placed mouse #6 of group C (6C) in the lid I bent over my bench and started the challenge of the already out cold mouse #5 of group C (5C). I wrapped him up and put my tools in the sterilizer. When I turned around, I discovered that mouse 1D had decided to go for a stroll.

I must have left a small gap in the wire lid of the cage, and he had stood up and wriggled through the little crack while my back was turned. For some strange reason, he crossed the bench and got into the upturned lid, the lid holding his cagemate in the altered state, 6C. He was right up on 6C, sniffing around. I picked him up and placed him back in the cage. I told him to take a time out.

After a minute, I took my tools out of the sterilizer and let them cool. I picked up the shaver and went to fetch 5C. Odd, his shoulder blade is wet. As I lifted him, I tested his withdrawl reflex....still intact. He's not out yet, give him a few more minutes. How'd this get wet, anyway......Oh, God.

1D had not been sniffing.

The wet fur was wet with blood. Near the base of the neck, a rather large patch of skin and fur were simply gone, and there were bite wounds on his flank. And under the wet patch of shoulder, a significant portion of flesh had been chewed away. But the real kicker is that the mouse wasn't all the way out yet. He wasn't moving on his own, but he was responding to painful stimuli (withdrawl reflex); he could feel what was happening to him, and could respond to pain by trying to pull away. I gave him a huge dose of anesthetic and put him out of his misery. I killed the offender in a similar manner. Preventative, not punitive. There are three other mice that are going to be living in that cage for the next week.  Although I must admit there is a part of me that wanted to give him a big test dose of bacteria. Say 10¹⁰ or so.

I am the one that hacks the mice into tiny bits, not you!

To note, I've been doing this kind of technique for about 5 years. I've been knocking mice out like this for other techniques for almost decade, and I have never seen a mouse try to eat another still-living mouse. I'm sure I could Google it in hindsight and find a dozen examples, but this is a first for me.