def _format_rxn_str_for_pdep(reaction, pressure='all'):
    """ Add the bath gas M species to the reaction string for
        pressure dependent reactions in the appropriate format.

        :param reaction: chemical equation for the reaction
        :type reaction: str
        :param pressure: signifies the level of pressure dependence
        :type pressure: str
        :return: three_body_reaction: chemical equation with M body
        :rtype: str
    """
    # Determine format of M string to be added to reaction string
    assert pressure in ('low', 'all')
    if pressure == 'all':
        m_str = ' (+M)'
    else:
        m_str = ' + M'

    # Add the M string to both sides of the reaction string
    [lhs, rhs] = reaction.split('=')
    three_body_reaction = lhs + m_str + ' = ' + rhs + m_str

    return three_body_reaction