If you're at all like me, you are reassured by hard numbers and measurables. This should help.
In this situation one of the likely pathogens would be one of the Salmonella species. Salmonella is killed by temperatures in excess of 130 F (55 C). However, it's not an instant death. The time to kill Salmonella decreases exponentially as the temperature increases.
The following table represents all the temperature and duration to kill 99.9999% of the most heat-resistant strain Salmonella senftenberg. This species is as much as 30x more heat resistant than a "normal" S. typhimurium.
Temperature | Time
---------------|------
140 F (60 C) | 60m
150 F (65 C) | 10m
160 F (70 C) | <2m
For any given temperature the proportion of bacteria killed is constant. 1/6th the time kills 90%, 1/3rd kills 99%, 1/2 kills 99.9% etc.
USDA guidelines to kill Salmonella and E. Coli are as follows:
Temperature | Time
---------------|------
135 F (57 C) | 86.4m
140 F (60 C) | 8.6m
145 F (63 C) | 2.7m
150 F (65 C) | 51.9s
160 F (70 C) | < 6s
165 F (74 C) | < 2s
So, needless to say, re-searing your steak at a typically high stove top temperature (at least 300 F [149 C]) for even a fraction of a second will result in utter devastation to whatever beastie population you may have picked up in recontaminating your meat. Chances are, if the steak came hot out of the pan, and you removed it from the plate quickly, the residual heat alone would be enough to kill it. The re-sear certainly doesn't hurt though.
A good rule of thumb is to wash plates as you go. If this is not possible or convenient you should minimally remove the plate from your vicinity by putting it in the sink or dishwasher as soon as you have taken the food off of it.
Butchered meat is generally sterile except on its exterior. (That doesn't mean parasite- or botulism-free, but it's a start.)
Get the best quality you can from a source you trust. Keep it at as low a temperature as possible, and don't expose it to warm air for more than the few minutes it takes to prepare.
Cut with a clean knife on a clean surface. Put it right back into the fridge at a very low temp.
Salt and acid, if you use them in your sauce, will provide some anti-bacterial benefits, but this is not foolproof.
People all over the world eat raw meat, but it's best to start with small amounts and see how your system handles it. If you're a child/pregnant/old/sick, it's much less of a good idea.
Best Answer
First, 145 °F (63 °C) and higher is the temperature for a well done steak. So, with the addition of carry-over cooking, your results don't surprise me. If you are shooting for rare, cook to an internal temperature of 125 °F (52 °C), and let your steak rest 10 minutes before slicing.
While the USDA correctly and necessarily provides temperature guidelines, in fact the reduction of pathogens follows a logarithmic curve and includes the variables of temperature and time. That means, in general, that longer times at lower temperatures will reduce pathogens. This understanding is the basis of sous vide cooking, for example.
Additionally, we generally assume that any potential pathogens are only present on the surface of whole muscle cuts. So, again, in general, achieving the target temperature on the surface eliminates the threat.
Finally, the quality and handling of the raw product is critical. It is important that you have fresh products, kept refrigerated or frozen until use, and handled by people who are practicing safe handling procedures (washed hands or gloves, ...).