\documentclass[a4paper]{article}

\usepackage{pgf}

\begin{document}

With the fix (after version 2.10):

$\pgfmathprintnumber{-1e5}$

\pgfmathprintnumber{-1e5}

Without the fix (2.10 and earlier):

\pgfkeys{/pgf/compat/number printer=2.10}

$\pgfmathprintnumber{-1e5}$

\pgfmathprintnumber{-1e5}
\end{document}

