latex command has extra space

Question

I'm writing a command to deal with bacteria names. They're written in full the first time, then with just the first name and a period following it. I've managed what I have by cobbling together others' examples, though I seem to have an extra space. I'm ignoring for now that sometimes the second name is shortened as well.

MWE

\documentclass[letterpaper,10pt]{scrartcl}
\usepackage{etoolbox}


\def\splitfirstchar#1#2\sentinel{#1.}
\newcommand{\first}[1]{\splitfirstchar#1\sentinel} 

\def\testwords#1{%
    \readwords#1\relax
}
\def\readwords#1 #2\relax{%
    \ifcsundef{bactused}{
        \def\bactused{true}%want full name
        \textit{#1 #2}
    }{
        \textit{\first{#1} #2}
    }
    %\relax
}
\def\ecoli{\testwords{Escherichia coli}}
\begin{document}
\ecoli
\ecoli\ecoli. Blah
\end{document}

Produces:

So where is the extra space coming from?

Answer 1

You need % at the ends of your lines that containt \textit{...} and the line that ends with {

Answer 2

TeX considers a <return> as whitespace just like it does a number of consecutive spaces. So, in your instance, there's actually a return after every use of \ecoli. The % "removes" this <return>.

From the TeX Book regarding the SHORT STORY by A. U. Thor (Chapter 6 Running TeX, p 26):

The % sign here is a feature of plain TeX that we haven't discussed before: It effectively terminates a line of your input file, without introducing the blank space that TeX ordinarily inserts when moving to the next line of input. Furthermore, TeX ignores everything that you type following a %, up to the end of that line in the file; you can therefore put comments into your manuscript, knowing that the comments are for your eyes only.