def parse_gram_stains(gram_file):
    """
    Produce a simple lookup dictionary for genus:gram_stain.
    """
    gram_stain = {}
    if not gram_file:
        return gram_stain
    with open(gram_file) as f:
        for line in f:
            genus, gram = line.strip().split()
            gram_stain[genus] = gram
    return gram_stain